Yes, "just for the halibut" is occasionally used as a humorous euphemism for "just for the hell of it," mainly because it almost rhymes. But what makes it amusing (as opposed to similar euphemisms, like "for the heck of it") is that a halibut is a type of fish, and quite often the person who says "just for the halibut" will follow up the statement with an assortment of really bad fish puns that also sound very close to other words. For example:
For example, "I stopped by Joe's Bar for a quick drink, just for the halibut. I had to repeat my order three times because Joe is hard of herring. I didn't stay long because all the noise was giving me a haddock." :splat: