To be lost in something is written 沉湎, using this character.
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history
湎 is not used alone in modern Chinese; it survives only bound in 沉湎 (to wallow in, to be sunk in — usually drink or indulgence). It is the water radical 氵 with the phonetic 面, originally picturing a face flushed and submerged by wine.
register
Literary and bound only; modern speech uses 沉迷 for this idea.
氵 is the left-side three-drop form of the water radical 水. Here it points to drink — being submerged by wine — giving the sense of drowning in indulgence.
面 gives the sound (miàn to miǎn) and adds an image — a drinker's face awash with wine. It is the standalone character for face, reused for its reading and a faint visual hint.
