'Cynical' meant 'dog-like' — from Greek 'kyon' (dog), kin to 'canine' and 'hound.'
Believing that people are motivated purely by self-interest; distrustful of human sincerity or goodness.
From Latin 'cynicus,' borrowed from Greek 'kynikos' (κυνικός, dog-like, currish, shameless), the adjective of 'kyōn' (κύων, dog), from PIE *ḱwón- (dog). The Cynics were a school of Greek philosophy founded by Antisthenes and made famous by Diogenes of Sinope (c. 412–323 BCE). The name may reference the gymnasium 'Kynosarges' (κυνοσαργές, the swift or white dog) where Antisthenes taught; or it may reflect the Cynics' deliberate identification with dogs — living in public without shame, biting social pretension, eating what they found, and sleeping
The PIE root *ḱwṓ (dog) connects 'cynical' to 'canine' (from Latin 'canīnus') and even 'hound' (from Proto-Germanic *hundaz). Diogenes, the most famous Cynic, embraced the 'dog' label — he ate, slept, and performed all bodily functions in public, arguing that if dogs are not ashamed of natural behavior, humans shouldn't be either.