early 15c., suppliaunt, "a petitioner at law," from Old French suppliant, variant of soupleiant, noun use of present participle of supplier "to plead humbly, entreat, beg, pray," (Old French souploier, 12c.), from Latin supplicare "beg, beseech" (see supplication). Originally in English especially at law; sense of "humble petitioner" is from mid-16c. As an adjective, "supplicating, entreating" from 1580s. Related: Suppliance; suppliantly.
1590s, "suppliant, applying to a higher authority," present-participle adjective from appeal (v.). The sense of "attractive" is attested by 1854. Related: Appealingly.
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.