Synonyms of 'humourless'
seriousDictionary, intense, solemn
He was a straight-faced, humourless character.
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Intense is used to describe something that is very great or extreme in strength or degree.
He was sweating from the intense heat.
If you describe someone as po-faced, you think that they are being unnecessarily serious about something.
a politically-correct bastion of po-faced self-righteousness
Thesaurus for humourless from the Collins English Thesaurus
If you accuse someone of being humourless , you mean that they are very serious about everything and do not find things amusing .