gullible (adj.)

1782, earlier cullibility (1728), probably from gull (n.2) "dupe, sucker" + -ability.

1690s, earlier cully (1660s) "a dupe, a sap-head," "a verdant fellow who is easily deceived, tricked, or imposed on" [Century Dictionary], rogues' slang, of uncertain origin.

Perhaps a shortening of cullion "base fellow," originally "testicle" (from French couillon, from Old French coillon "testicle; worthless fellow, dolt," from Latin coleus, literally "strainer bag;" see cojones). Another theory traces it to Romany (Gypsy) chulai "man." Also sometimes in the form cully, however some authorities assert cully was the canting term for "dupe" and cull was generic "man, fellow" without implication of gullibility. Compare also gullible. Related: Cullibility (1728).