reef (n.1)

"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)

"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.)

1660s, "take in, roll up" (as one would a section of a sail on a ship), from reef (n.2). Related: Reefed; reefing.

Others are reading

Definitions of reef from WordNet
reef (v.)
lower and bring partially inboard;
reef the sailboat's mast
reef (v.)
roll up (a portion of a sail) in order to reduce its area;
reef (v.)
reduce (a sail) by taking in a reef;
reef (n.)
a submerged ridge of rock or coral near the surface of the water;
reef (n.)
one of several strips across a sail that can be taken in or rolled up to lessen the area of the sail that is exposed to the wind;
Reef (n.)
a rocky region in the southern Transvaal in northeastern South Africa; contains rich gold deposits and coal and manganese;
Synonyms: Witwatersrand / Rand