I'm not certain, but I think 'halibut' is a euphemism for 'hell of it' in 'Just for the hell of it":
Also, for the heck of it
; just for the hell of it
. For no particular reason; on a whim. For example, We drove by the old place just for the hell of it. In the first variant, heck is a euphemism for hell. [First half of 1900s] Also see for fun, def. 2.
The American Heritage® Dictionary of Idioms
by Christine Ammer. Copyright © 2010, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.