Halibut

google

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late Middle English: from haly ‘holy’ + obsolete butt ‘flatfish’ (because it was often eaten on holy days).

wiktionary

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From Middle English *halibutt, equivalent to holy +‎ but(“flatfish”), since the fish was often eaten on holy days. Compare Dutch heilbot, German Heiligbutt, Heilbutt, Heilbutte. Compare also Danish helleflynder, Swedish helgeflundra.

etymonline

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halibut (n.)

large flatfish, early 15c., perhaps from hali "holy" (see holy) + butte "flatfish" (see butt (n.4)). Supposedly so called from its being eaten on holy days (compare cognate Dutch heilbot, Low German heilbutt, Swedish helgeflundra, Danish helleflynder).