source: Speedwagon withdraws coolly

slip away

depart without saying goodbye; leave quietly or surreptitiously.

(気づかれないように)そっと立ち去る

slowly disappear; recede or dwindle:

〈感情･能力･機会などが〉(次第に)無くなる

e.g. his ability to concentrate is slipping away.

die peacefully (used euphemistically):

(老衰のため安らかに)逝く, 死ぬ (!dieの婉曲表現)

e.g. he lay there and quietly slipped away.