To tuck in is to push the bottom of a shirt inside your waistband. Tuck in your shirt, tucked in neatly, tuck it in.
It also means to start eating heartily, as in tuck in everybody, and to settle a child in bed by folding the covers under. All three come from the same plain sense of folding something inward.
1phrasetuck inTo push the bottom of a shirt inside your waistband.
“Tuck in your shirt before the interview, or you'll look scruffy.”
tuck in is a folding.
From tuck, to fold or gather in, plus in.
See tuck (1).
Etymology from Wiktionary, CC BY-SA
Practised in Clothes & Fashion