Collins COBUILD Advanced Learner's English Dictionary - tuck in

1. If you tuck in a piece of material, you keep it in position by placing one edge or end of it behind or under something else. For example, if you tuck in your shirt, you place the bottom part of it inside your trousers or skirt. ‘Probably,’ I said, tucking in my shirt... Tuck the sheets in firmly. PHRASAL VERB: V P n (not pron), V n P 2. If you tuck a child in bed or tuck them in, you make them comfortable by straightening the sheets and blankets and pushing the loose ends under the mattress. I read Lili a story and tucked her in her own bed... My mother would tuck me in, turn out the lights and tiptoe out. PHRASAL VERB: V n P n, V n P