“shelf” – English explanatory dictionary

noun a flat piece of wood attached to a wall or in a cupboard on which things can be put ○ He put up or built some shelves in the kitchen. ○ The shelves were packed with books. ○ Put that book back on the shelf. ○ Can you reach me down the box from the top shelf? ○ The plates are on the top shelf in the kitchen cupboard. □ on the shelf still not married (informal.) ○ She thought she was on the shelf at thirty-five, and then Mr Right came along.

“on the SHELF,” not yet disposed of; young ladies are said to
be so situated when they cannot meet with husbands. “On the SHELF” also
means pawned, or laid by in trust.

n.; pl. Shelves (#). [OE. shelfe, schelfe, AS. scylfe; akin to G. schelfe, Icel. skjalf. In senses 2 & 3, perhaps a different word (cf. Shelve, v. i.).]
1. (Arch.) A flat tablet or ledge of any material set horizontally at a distance from the floor, to hold objects of use or ornament.
2. A sand bank in the sea, or a rock, or ledge of rocks, rendering the water shallow, and dangerous to ships. “On the tawny sands and shelves.” Milton. “On the secret shelves with fury cast.” Dryden.
3. (Mining) A stratum lying in a very even manner; a flat, projecting layer of rock.
4. (Naut.) A piece of timber running the whole length of a vessel inside the timberheads. D. Kemp. To lay on the shelf, to lay aside as unnecessary or useless; to dismiss; to discard.