In Chinese, how can I describe someone as being cynical? cynical as in "believing the worst of human nature and motives; having a sneering disbelief in e.g. selflessness of others."
There are several words with close meaning: 愤世嫉俗、犬儒主义。

犬儒主义 is most close to the word as Cynic has the meaning of 犬 in ancient Greek and Chinese. And one of the representative Greek philosophers is Diogenēs o Sinopeus who proposed the philosophy which is very similar to "老庄哲学" in ancient China.

However, "犬儒主义" is not a negative emotional word -- not so discouraging -- it describes someone who lives and satisfies to his own life style which is different from the rest part of the "incorrigible" world.