Fishing for word origins

// I came across a reference to halibut last night, and realized it was a funny-looking word.To the OED!

I figured it was some culture's name for the "large flat fish (Hippoglossus vulgaris), abundant in the northern seas," but it turns out this is the etymology:

[app. f. haly, HOLY + BUTT n.1 flat fish: cf. mod.Du. heilbot (in Kilian heylbot, eelbot), LG. heilbutt, heilige butt, Norse heilag-fiski, Sw. helgeflundra, Da. helleflynder i.e. holy flounder: supposed to be so called from being so commonly eaten on holy-days.]

The OED said butt, used in this sense, is a word of "obscure origin" that means a flat fish. So halibut means holy flat fish. Batman. (Sorry. I couldn't help it.)