From Middle English gayn, gain, gein (“profit, advantage”), from Old Norse gagn (“benefit, advantage, use”), from Proto-Germanic *gagną, *gaganą (“gain, profit", literally "return”), from Proto-Germanic *gagana (“back, against, in return”), a reduplication of Proto-Germanic *ga- (“with, together”), from Proto-Indo-European *ḱóm (“next to, at, with, along”). Cognate with Icelandic gagn (“gain, advantage, use”), Swedish gagn (“benefit, profit”), Danish gavn (“gain, profit, success”), Gothic 𐌲𐌰𐌲𐌴𐌹𐌲𐌰𐌽 (gageigan, “to gain, profit”), Old Norse gegn (“ready”), dialectal Swedish gen (“useful, noteful”), Latin cum (“with”); see gain-, again, against. Compare also Middle English gaynen, geinen (“to be of use, profit, avail”), Icelandic and Swedish gagna (“to avail, help”), Danish gavne (“to benefit”).

The Middle English word was reinforced by Middle French gain (“gain, profit, advancement, cultivation”), from Old French gaaing, gaaigne, gaigne, a noun derivative of gaaignier (“to till, earn, win”), from Frankish *waidanjan (“to pasture, graze, hunt for food”), ultimately from Proto-Germanic *waiþiz, *waiþō, *waiþijō (“pasture, field, hunting ground”); compare Old High German weidōn, weidanōn (“to hunt, forage for food”) (Modern German Weide (“pasture”)), Old Norse veiða (“to catch, hunt”), Old English wǣþan (“to hunt, chase, pursue”). Related to wathe, wide.

gain (third-person singular simple present gains, present participle gaining, simple past and past participle gained)

(transitive) To acquire possession of.

(transitive, dated) To come off winner or victor in; to be successful in; to obtain by competition.

to gain a battle; to gain a case at law

(transitive) To increase.

(intransitive) To be more likely to catch or overtake an individual.

I'm gaining (on you).

gain ground

(transitive) To reach.

to gain the top of a mountain