lie down, sleep, rest, bed, remain unsold
寝 centers on lying down to rest or sleep, and by extension a bed or place for sleeping. The idea of remaining unsold is a figurative application of something lying dormant or unused.
寝 is the modern simplified form of 寢. The older form combines 宀 (roof) with 爿 (bed) and 㑴 (phonetic), suggesting sleeping or lying down indoors. The modern form replaces the lower part with a simplified shape.
Under a roof 宀, a bed 丬 is set up for resting. The lower right part looks like a person settling in, so 寝 means to lie down and sleep.
For シン, imagine a person lying down in bed, sinking into a deep sleep: sink -> シン, and the bed is under the roof.
sleep
to sleep (lying down)
(afternoon) nap; sleeping during the day; siesta
bedroom
to put to bed; to let sleep
going to bed; retiring (for the night)
sleeping in late; oversleeping
lack of sleep; insufficient sleep
bed; couch
talking in one's sleep; somniloquy
bed; berth; crib; cot; bedroom
turning over in bed; rolling over; tossing and turning
bedding; bedclothes
go to bed early
to put to bed; to let sleep
sleeping face
staying the night; lodging (at)
breathing of a sleeping person
sleeping posture
to change sides; to double-cross; to betray
nightclothes; nightwear; sleepwear; pyjamas; pajamas; nightgown; nightdress
sleeping late in the morning; sleeping in; lie-in; oversleeping
unable to sleep well
to go to bed; to go to sleep; to fall asleep