re- (expressing intensive force) + manere ‘to stay’

source: [ピザの残った一切れ｜無料GIF画像検索 GIFMAGAZINE 1295666]

continue to exist, especially after other similar or related people or things have ceased to exist:

〈物･事が〉 «…に» 残っている(｟よりくだけて｠ be left) «of»

e.g. a cloister is all that remains of the monastery.

stay in the place that one has been occupying:

〖remain＋副詞〗 〈人･物･事が〉…にとどまる, 居残る (!(1)〖副詞〗は場所･状態の表現. (2)stayよりややかたく, ｟書｠ で好まれる)

e.g. her husband remained at the beach condo.

with complement continue to possess a particular quality or fulfill a particular role:

〖remain C〗 〈人･物･事が〉C〈状態〉のままである, 相変わらずCだ (!Cは〖名詞〗〖形容詞〗〖分詞〗など)

e.g. he had remained alert the whole time.

be left over after others or other parts have been completed, used, or dealt with:

〖～ to be done〗 〈事が〉これから…されなければならない, まだ…されないままだ

e.g. several years remain in the transition period.

used to express the notion that something is not yet known or certain:

e.g. she has broken her leg, but it remains to be seen how badly.

late Middle English: from Old French remain-, stressed stem of remanoir, from Latin remanere, from re- (expressing intensive force) + manere ‘to stay’.