reef (n.1) Look up reef at Dictionary.com
"rock ridge underwater," 1580s, riffe, probably via Dutch riffe, from a Scandinavian source such as Old Norse rif "ridge in the sea; reef in a sail," literally "rib" (see rib (n.)).
reef (n.2) Look up reef at Dictionary.com
"horizontal section of sail," late 14c. (mid-14c. in rif-rope), from a Scandinavian source such as Old Norse rif "reef of a sail," probably a transferred use of rif "ridge under the sea; rib" (see rib (n.) and compare reef (n.1)). German reff, Swedish ref, Norwegian riv, Danish reb likely all are from the Old Norse word.
reef (v.) Look up reef at Dictionary.com
1660s, "take in, roll up" (as a sail on a ship), from reef (n.2). Related: Reefed; reefing.