noun
permanently laid-out futon; bedding never put away
Refers to a futon or bedding that is left out all day instead of being folded and stored. Often carries a nuance of laziness or a relaxed lifestyle.
彼の部屋は万年床で、いつも布団が敷きっぱなしだ。
His room has a permanently laid-out futon; the bedding is always left out.
万年床はダニの温
A futon that is never put away easily becomes a breeding ground for dust mites.