The base meaning of cynic is "like a dog." The term was applied to the followers of Antisthenes, who taught a Zen-like detachment from desire, probably because Kyon (or "dog") was the nickname of Diogenes, the most famous cynic of all.
Nowadays the word often means "doubtful" or "full of disbelief" concerning the claims made about the virtues of a particular thing.
Example: That TV advert seems too good to be true, so I am cynical about whether the device works.