shelf n.1

late 14c., "thin slab or plank fixed horizontally to a wall or frame and used for supporting small objects; a transverse board in a case or cabinet," perhaps from Middle Low German schelf "shelf, set of shelves," or perhaps from Old English cognate scylfe, which could have meant "shelf, ledge, floor, deck of a ship" (the sense is uncertain), and scylf "peak, pinnacle."

These all are from Proto-Germanic *skelf- "split," possibly from the notion of a split piece of wood (compare Old Norse skjölf "bench"), from an extended form of the PIE root *skel- (1) "to cut," and thus distantly related to shell, etc.

From mid-15c. as "grassy bank." The meaning "ledge of rock" (as in later continental shelf) is from 1809, perhaps from or influenced by shelf (n.2).

By 1920s in reference to the display of goods in a shop, hence shelf life "time goods may be kept or stored unsold before they begin to spoil" (1927). The figurative phrase on the shelf "out of the way, inactive" is attested from 1570s (also used 19c. of unmarried women with no prospects). Off the shelf "ready-made, from a supply of ready-made goods" is from 1936. Related: Shelves.

"sandbank, underwater ridge," 1540s, a word of obscure origin; evidently identical to Middle English shelp "sandbar in a river" (early 15c.), but the sound shift is unexpected. Shelp might be from Old English scylp "crag" or Middle Dutch schelp-. Also sometimes regarded as a particular use of shelf (n.1) or from the verbs shelve. Related: Shelfy "abounding in sandbanks."