adj. — Newfoundland
of the water: rough from wind or storm; choppy.
Type: 2. Preservation — Loppy is an adjective that derives from "lop", a noun used to indicate 'a state of the sea when the waves are short and lumpy' (OED-3, s.v. "lop" n.6). Because a great number of English immigrants settled in Newfoundland and Labrador in the 18th and 19th centuries, the term is almost certainly preserved from British English (EDD, s.v. "loppy"). According to OED-3 (s.v. "lop" n.6), the etymology of loppy might possibly include some onomatopoeic element and could also be related to the verb "lap", as in 'to lap up water from a dish'.
See also DNE, s.v. "loppy".