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"rock ridge underwater," 1580s, riffe, probably via Dutch riffe, from a Scandinavian source, cf. Old Norse rif "ridge in the sea; reef in a sail," literally "rib" (see rib (n.)).
"horizontal section of sail," late 14c. (mid-14c. in rif-rope), from a Scandinavian source, cf. Old Norse rif "reef of a sail," probably a transferred use of rif "ridge under the sea; rib" (see rib (n.) and cf. reef (n.1)). German reff, Swedish ref, Norwegian riv, Danish reb likely all are from the Old Norse word.
1660s, "take in, roll up" (as a sail on a ship), from reef (n.2). Related: Reefed; reefing.
A strip or ridge of rocks, sand, or coral that rises to or near the surface of a body of water. See more at coral reef.