a thin flat usu long and narrow piece of a solid material fastened horizontally at a distance from the floor, e.g. on a wall or in a cupboard, bookcase, etc, to hold objects
something resembling a shelf in form or position, e.g.:
a partially submerged sandbank or ledge of rocks
a flat projecting layer of rock
= continental shelf
off the shelf
available from stock, not made to order
on the shelf
in a state of inactivity or uselessness
considered unlikely to marry, esp because of being too old
[Middle English schelfe from early Low German schelf]