Emily Dickinson Lexicon

REEF, n.¹ [D. reef; Dan. riv or rift; Sw. ref. These words coincide in orthography with the verb to rive, and if from this root, the primary sense is a division, W. rhiv and rhif. But in Welsh, rhêv signifies a collection or bundle, as thick; rhevu, to thicken in compass; and if from this root a reef is a fold, and to reef is to fold.]

A certain portion of a sail between the top or bottom and a row of eyelet holes, which is folded or rolled up to contract the sail, when the violence of the wind renders it necessary. – Mar. Dict.