Is it ensure, insure, or assure?
late 14c., from Middle Low German schelf "shelf, set of shelves," or from Old English cognate scylfe, which perhaps meant "shelf, ledge, floor," and scylf "peak, pinnacle," from Proto-Germanic *skelf- "split," possibly from the notion of a split piece of wood (cf. Old Norse skjölf "bench"), from PIE root *(s)kel- (1) "to cut, cleave" (see scale (n.1)).
Shelf life first recorded 1927. Phrase on the shelf "out of the way, inactive" is attested from 1570s; of unmarried women with no prospects from 1839. Off the shelf "ready-made" is from 1936. Meaning "ledge of rock" is from 1809, perhaps from or influenced by shelf (n.2). Related: Shelves.
"sandbank," 1540s, of unknown origin. Related: Shelfy "abounding in sandbanks."
See continental shelf.