As a standalone noun, 枕 is literary. In everyday speech, 枕头 (zhěntou) is the standard word for pillow. 枕 as a verb means to use something as a pillow: 枕着书睡觉 (sleeping with a book as a pillow).
Left wood radical — pictograph of a tree with branches and roots. The indexing radical: traditional Chinese pillows were wooden blocks, not soft cushions — carved porcelain or hardwood headrests that supported the neck while preserving the elaborate hairstyle. Wood is the material, baked into the character.
Right 冘 supplies the sound — yín drifting to zhěn through historical change. 冘 originally pictured a person carrying a load and pausing under its weight, hinting at the heavy-headed, settled-down sense of resting one's head. The same phonetic appears in 沉 to sink and 鸩 a poisonous bird.
