simp (n.)

"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.