a sum of the meanings of its morphemes: un/eat/able = "not fit to eat" where not stands for un- and fit for -able.
There are numerous derived words whose meanings can really be easily deduced from the meanings of their constituent parts. Yet, such cases represent only the first and simplest stage of semantic readjustment within derived words. The constituent morphemes within derivatives do not always preserve their current meanings and are open to subtle and complicated semantic shifts.
Let us take at random some of the adjectives formed with the same productive suffix -y, and try to deduce the meaning of the suffix from their dictionary definitions:
brainy (inform.) — intelligent, intellectual, i. e. characterised by brains
catty — quietly or slyly malicious, spiteful, i. e. characterised by features ascribed to a cat
chatty — given to chat, inclined to chat
dressy (inform.) — showy in dress, i. e. inclined to dress well or to be overdressed
fishy (e. g. in a fishy story, inform.) — improbable, hard to believe (like stories told by fishermen)
foxy — foxlike, cunning or crafty, i. e. characterised by features ascribed to a fox
stagy — theatrical, unnatural, i. e. inclined to affectation, to unnatural theatrical manners
touchy — apt to take offence on slight provocation, i. e. resenting a touch or contact (not at all inclined to be touched)1
The Random-House Dictionary defines the meaning of the -y suffix as "characterised by or inclined to the substance or action of the root to which the affix is at-
1 Some of the listed adjectives have several meanings, but only one is given so as to keep the list manageable.