寝棺 is read as ねかん (nekan) and means coffin.
ねかん
nekan
ねいりばな
when one has just fallen asleep
ねすがた
one's figure during sleep
ねげしょう
cosmetics put on before going to bed
しんにつく
to go to bed
ねたばをあわす
to hone one's blade
ねるまもおしんで
without sparing time for sleep
せきかん
sarcophagus
かん
coffin
ねだい
bed
ねまき
nightclothes
しん
sleep
しんしつ
bedroom