cant term for "dupe, sucker, credulous person," 1590s, of uncertain origin. Perhaps from verb meaning "to dupe, cheat" (see gull (v.)). Or it is perhaps from (or influenced by) the bird name (see gull (n.1)); in either case with a sense of "someone who will swallow anything thrown at him." Another possibility is Middle English gull, goll "newly hatched bird" (late 14c.), which is perhaps from Old Norse golr "yellow," from the hue of its down.
word-forming element expressing ability, fitness, or capacity, from Latin -abilitas, forming nouns from adjectives ending in -abilis (see -able). Not etymologically related to ability, though popularly connected with it.