gain (n.) Look up gain at
late 15c., "that which has been acquired" (possessions, resources, wealth), from Middle French gain, from Old French gaaigne "gain, profit, advantage; work, business; booty; arable land" (12c.), from gaaignier "to gain, earn; capture, win" (see gain (v.)). Meaning "any incremental increase" (in weight, etc.) is by 1851. Related: Gains.

The original French word enfolded the notions of "profit from agriculture" and "booty, prey." Neither the verb nor the noun gain is in Middle English, which however had gainage "profit derived from agriculture" (late 14c., from Old French gaaignage); gaineier "farmer" (late 13c. as a surname); gainerie "a farm" (mid-15c.).
gain (v.) Look up gain at
1520s, "obtain as profit," from Middle French gagner, from Old French gaaignier "to earn, gain; trade; capture, win," also "work in the fields, cultivate land," from Frankish *waidanjan "hunt, forage," also "graze, pasture," from Proto-Germanic *waithanjan "to hunt, plunder," from *waithjo- "pursuit, hunting" (cognates: Old English waþ "hunting," German Weide "pasture, pasturage," Old Norse veiðr "hunting, fishing, catch of fish"), from PIE *weie- "to strive after, pursue with vigor, desire" (see venison). Meaning "obtain by effort or striving" is from 1540s; intransitive sense of "profit, make gain" is from 1570s. Meaning "arrive at" is from c. 1600. Of timepieces by 1861. Related: Gained; gaining. To gain on "advance nearer" is from 1719. To gain ground (1620s) was originally military.