'Pessimistic' just means expecting that something bad/unfavorable will happen. A person could be pessimistic about the likelihood that their favorite team will win the game, the weather being rainy/unpleasant, or have reduced expectations about their future job prospects. Cynicism is more based upon not believing in anything that allegedly has an ethical/moral foundation. Cynical people assume that everything is done based on self-interest, for profit, to make money, etc. It is generally having to do with intent or motive.