"well-formed and agile," 1886, probably a back-formation from ungainly. Earlier "ready, prompt" (1620s), from gain (n.).
Entries linking to gainly
ungainly adj.
1610s, "unfit, improper," from Middle English ungeinliche, from ungein (late 14c.) "inconvenient, disagreeable, troublesome," from un- (1) "not" + gein "kind, helpful; reliable; beneficial; suitable, appropriate; convenient," from Old Norse gegn "straight, direct, helpful," from Proto-Germanic *gagina "against" (see again). Old English had ungænge "useless, vain."
gain n.
c. 1200, gein, "advantage, benefit; help," c. 1300, "reward, profit, that which has been acquired" (possessions, resources, wealth), from Old French gain, gaaigne "gain, profit, advantage; work, business; booty; arable land" (12c.), from Germanic, and from Old Norse (see gain (v.)). Meaning "any incremental increase" (in weight, etc.) is by 1851. Related: Gains. The French word enfolded the notions of "profit from agriculture" and "booty, prey."