Why is the ninth month called September?
late 15c., from Middle French gain, from Old French gaaigne "gain, profit, advantage; booty; arable land" (12c.), from gaaignier "to gain" (see gain (v.)). The original French sense enfolded the notions of "profit from agriculture" and "booty, prey." Implied earlier in Middle English gaignage (late 14c.) "profit from agriculture."
1520s, 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 *wartho "hunting ground" (cf. Old English waþ "hunting," German Weide "pasture, pasturage," Old Norse veiðr "hunting, catch of fish"), from PIE *weie- "to strive after, pursue with vigor, desire" (see venison). Related: Gained; gaining. To gain on "advance nearer" is from 1719. To gain ground (1620s) was originally military.
An increase in amount or degree.