Synonyms of 'unpremeditated'
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He made some heated and extempore remarks.
An impromptu action is one that you do without planning or organizing it in advance.
They put on an impromptu concert for the visitors.
If you describe someone as impulsive, you mean that they do things suddenly without thinking about them carefully first.
He is too impulsive to be a responsible prime minister.
Thesaurus for unpremeditated from the Collins English Thesaurus
not planned beforehand ; spontaneous