脱線
だっせん · dassen · JLPT N2 · noun
digression
🚂 脱 = off, 線 = line: a train that comes off its track, so the talk goes off track
脱 means leave or slip out of, and 線 means line or track. A derailed train leaves its rails, just as a conversation leaves its topic in a digression.
脱 — flesh radical 月 (肉) + 兑 tuì (phonetic)
脱 is a semantic-phonetic compound. The flesh radical 月 (from 肉) signals the body, while 兑 supplies the sound. Originally it meant shedding flesh or skin, like a snake molting, and broadened to mean 'to take off, slip off, or escape.'