the.com/water bed

means A mattress filled with water instead of springs or foam, so lying down means gently sloshing rather than sinking still.

from A plain compound of 'water' + 'bed,' describing exactly what it is. The modern design was patented in the 1960s, often credited to a design student's project, though water-filled bedding concepts had been tinkered with earlier for medical and comfort purposes — invalid care in particular, since a water mattress distributes weight evenly and resists bedsores. The 1970s turned it from medical gadget into a symbol of swinging bachelor-pad culture, which is a social fact, not an etymological one — the word itself never strayed from its literal roots.