English Thesaurus
(adjective) 
Definition
lacking in physical or mental strength
I was too weak to move my arms and legs.
Synonyms
feeble
He was old and feeble.
,
exhausted
She was too exhausted even to think clearly.
,
frail
She lay in bed looking particularly frail.
,
debilitated
, ,
wasted
exercises designed to help build up his wasted limbs
,
weakly
,
tender
The newborn looked so fragile and tender.
,
delicate
He was a physically delicate child.
, , ,
shaky
Our house will remain on shaky foundations unless the architect sorts out the basement.
,
sickly
He had been a sickly child.
,
languid
He's a large languid man with a round and impassive face.
,
puny
Our Kevin has always been a puny lad.
,
decrepit
a decrepit old man
, ,
infirm
her ageing, infirm husband
,
anaemic
Losing a lot of blood makes you tired and anaemic.
,
effete
the sun-starved looks of an effete intellectual
,
enervated
Warm winds make many people feel enervated and depressed.
Opposites
(adjective) 
Definition
(of a part of the body) not functioning as well as is normal
His eyesight had always been weak.
Synonyms
deficient
deficient landing systems
,
wanting
She examined his work and found it wanting.
,
poor
poor wages and terrible working conditions
A poor crop has sent vegetable prices spiralling.
,
lacking
Why was military intelligence so lacking?
,
inadequate
She felt quite painfully inadequate in the crisis.
,
pathetic
That's the most pathetic excuse I've ever heard.
,
faulty
Their interpretation was faulty.
, ,
under-strength
Opposites
(adjective) 
Definition
lacking in moral strength
a clash between a weak minister and a domineering civil servant
Synonyms
ineffectual
The ill-advised and ineffectual policies
, ,
cowardly
I was too cowardly to complain.
,
powerless
His leg muscles were powerless with lack of use.
,
soft
He says the measure is soft and weak on criminals.
,
impotent
Bullies can leave people feeling hurt, angry and impotent.
,
indecisive
He was criticised as a weak and indecisive leader.
,
infirm
She has little patience with the `infirm of purpose'.
,
spineless
bureaucrats and spineless politicians
,
boneless
,
timorous (literary)
He is a reclusive, timorous creature.
,
weak-kneed (informal)
,
namby-pamby
I despise his wimpy, namby-pamby attitude.
,
irresolute
They launched the attack for fear of seeming irresolute.
Opposites
(adjective) 
He managed a weak smile and said, `Don't worry about me.'
Synonyms
slight
It's only made a slight difference.
,
faint
She made a faint attempt at a laugh.
,
feeble
This is a feeble argument.
, ,
shallow
I think he is shallow, vain and untrustworthy.
,
(adjective) 
Definition
lacking strength or power
Her voice was so weak we could hardly hear her.
Synonyms
faint
He became aware of the soft, faint sounds of water dripping.
,
soft
When he woke again he could hear soft music.
She spoke in a soft whisper.
,
quiet
A quiet murmur passed through the classroom.
, ,
small
a very small voice
,
low
Her voice was so low he had to strain to catch it.
, , , ,
muffled
She heard a muffled cough behind her.
,
imperceptible
His hesitation was almost imperceptible.
Opposites
(adjective) 
Definition
liable to collapse or break
The animals escaped through a weak spot in the fence.
Synonyms
fragile
Coffee was served to them in cups of fragile china.
,
brittle
Pine is brittle and breaks easily.
,
flimsy
a flimsy wooden door
,
unsound
He was rejected as an army conscript as being of unsound mind.
,
fine
She wears fine jewellery wherever she goes.
,
delicate
Although the material looks tough, it is very delicate.
,
frail
The frail craft rocked as he clambered in.
,
dainty
a pair of the daintiest little kid slippers
,
breakable
My parents had a remarkable array of breakable objects.
(adjective) 
Definition
lacking in importance, influence, or strength
The trade unions are in a very weak position.
Synonyms
unsafe
In the larger neighbourhood, I felt very unsafe.
,
exposed
The troops are exposed to attack by the enemy.
,
vulnerable
criminals who prey on the more vulnerable members of our society
,
helpless
The children were left helpless.
,
wide open
The virus leaves the body wide open to infection.
,
unprotected
an unprotected girl, going along that river walkway in the dark
,
untenable
He claimed the charges against him were untenable.
,
defenceless
a leader who would leave the country isolated and defenceless
,
unguarded
The U-boat entered in through a narrow unguarded eastern entrance.
Opposites
, , ,
well-defended
(adjective) 
Definition
not convincing
The evidence against him was too weak to hold up in court.
Synonyms
unconvincing
He was given the usual unconvincing excuses.
,
unsatisfactory
The inspectors said a third of lessons were unsatisfactory.
,
lame
He mumbled some lame excuse about having gone to sleep.
,
invalid
Those arguments are rendered invalid by the hard facts.
,
flimsy
The charges were based on flimsy evidence.
, ,
Opposites
(adjective) 
Definition
not having a strong flavour
a weak cup of tea
Synonyms
tasteless
The fish was mushy and tasteless.
,
thin
The soup was thin and clear.
,
diluted
Encourage your child to drink diluted fruit juice.
,
watery
a plateful of watery cabbage soup
,
runny
Warm the honey until it becomes runny.
,
insipid
It tasted bland and insipid, like warm cardboard.
,
wishy-washy (informal)
,
under-strength
,
milk-and-water
He dismissed the project as `milk and water'.
,
waterish
Opposites
, , , ,
flavoursome
,
industrial-strength mainly humorous)
10 (adjective) 
Definition
lacking strength or power
The light was so weak we could barely see anything
Synonyms
dim
She stood waiting in the dim light.
,
pale
A pale light seeped through the window.
, , , ,
dull
The box closed with a dull thud.
,
wan
The lamp cast a wan light through the swirls of fog.

quotation

The weakest goes to the wall [William ShakespeareRomeo and Juliet]

Additional synonyms

in the sense of anaemic
Definition
pale and sickly-looking
Losing a lot of blood makes you tired and anaemic.
Synonyms
wan,
characterless,
like death warmed up (informal)
in the sense of breakable
My parents had a remarkable array of breakable objects.
in the sense of brittle
Definition
easily cracked or broken
Pine is brittle and breaks easily.
Synonyms

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Video: pronunciation of 'weak'

Source

Thesaurus for weak from the Collins English Thesaurus
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