reef

Reef comes from Old Norse rif, meaning both 'rib' and 'reef,' tracing back to Proto-Germanic *ribją and further to a Proto-Indo-European root meaning 'arch, ceiling, cover.' A second sense of reef, used for folding part of a sail, likely comes from the same Old Norse rif, as a figurative extension of 'rib' — though it may instead trace to a different Old English word, rift, meaning 'curtain, veil.'

The word began as a term for a rib, then extended figuratively — a curved ridge of coral or rock could be seen as the sea's rib. That same imagery may explain why sailors also used 'reef' for the ribbed folds taken in when shortening a sail.

Reef is a doublet of rib — the two words share the same Old Norse ancestor, rif, just taking different paths into English.