Kelp

google

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late Middle English: of unknown origin.

wiktionary

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14th c., from Middle English culp, culpe, kilp, but of unknown ultimate origin.

etymonline

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kelp (n.)

1660s, "large seaweed," from Middle English culpe (late 14c.), a word of unknown origin. Specifically of a type of Pacific seaweed of the Americas from 1834. Kelper "native or inhabitant of the Falkland Islands" is attested from 1896.