In nautical terms, to reduce the area of a sail by rolling or folding part of it.
Variant or dialectal form related to 'reef', from Old Norse 'rif' meaning to fold or roll. The 'bag-' prefix might relate to the bunched appearance of reefed sail.
Sailors developed a complex vocabulary for every tiny adjustment of rope and fabric—'bagreef' is one of hundreds of nautical terms that show how specific knowledge demands specific language.
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