c. 1300, "soft, tender," from Old French souple, sople "pliant, flexible; humble, submissive" (12c.), from Gallo-Roman *supples, from Latin supplex "submissive, humbly begging, beseeching, kneeling in entreaty, suppliant," literally "bending, kneeling down," perhaps an altered form of *supplacos "humbly pleading, appeasing," from sub "under" (see sub-) + placare "to calm, appease, quiet, soothe, assuage," causative of placere "to please" (see please, and compare supplication).
Meaning "pliant" is from late 14c.; figurative sense of "artfully obsequious, capable of adapting oneself to the wishes and opinions of others" is from c. 1600. Supple-chapped (c. 1600) was used of a flatterer. Related: Suppleness.