"person of limited or feeble intelligence," 1640s, according to OED probably a jocular formation from simple and -ton, suffix extracted from surnames (and ultimately place-names). Compare skimmington, personification of an ill-used spouse, c. 1600; OED compares idleton from the English Dialect Dictionary. Century Dictionary sees it as a French diminutive of simplet, though no such diminutive had yet turned up. Also compare the -by terminations under rudesby.

1903, circus slang shortening of simpleton.

1876, "single card of a suit in a hand; a hand containing only one card of a suit," originally in whist, from single (adj.); compare simpleton, etc. Extended early 20c. to other instances of singularity.

"clown, clodhopper," 1530s, from clump (n.), probably on model of simpleton.

also ninny-hammer, "simpleton," 1590s, from ninny + hammer (n.), but the signification of the second element is obscure.

1884, from tommy in sense of "a simpleton" (1829), diminutive of Tom (as in tom-fool) + rot (n.).

"stupid person, dunce, simpleton," mid-15c. (but early 13c. as a surname), from dull (adj.) + -ard.

1833, originally "the stuff they feed fools on" [Marryat]; probably an arbitrary formation from elements meant to sound ridiculous, perhaps with allusions to flap "a stroke, blow" and doodle "fool, simpleton."

1580s, "simpleton, stupid person," of unknown origin. Applied by sailors to the albatross and similar big, clumsy birds (1839). Related: Gony-bird.

"fool, simpleton," 1825, perhaps from gawp (v.) "to yawn, gape" (as in astonishment), which is attested from 1680s, a dialectal survival of galp (c. 1300), which is related to yelp or gape and perhaps confused with or influenced by gawk.