He tucked the letter into his pocket and quietly left.
Tips
usage
Read yē for the everyday 'tuck in' sense, as in 掖被子 (tucking in a blanket). A separate reading yè means 'to support by the arm; to help, foster' and also appears in the place name 张掖 (a city in Gansu).
夜 supplies the reading, shifting from yè to yē. The same phonetic surfaces in 液 (liquid) and 腋 (armpit); the armpit link is especially close, since 腋 marks the body part and 掖 the action of stowing something there.