The adjective "pliant" comes from 14th century Middle English via Old French ploiant, meaning "bending",  present participle of ploiier ("to bend, fold"), which itself derives from the Latin word plicāre ("to fold, bend, lay") from Proto-Indo-European root *plek- ("to plait").  It entered English describing physical flexibility, (like a young tree), and later extending to people or attitudes that are yielding, compliant, or adaptable.  It shares roots with words like "ply," "pleat," and "complicate".