I think this word is rooted in the French "beaucup" as in "merci beaucup" (if my French is misspelled, please pardon, as I don't have a refernce book in my office). I have heard "bookoo" used mostly (and heavily) by Vietnam vets who may have picked it up from the residue of French language leftover in Vietnam during the war years. So: "Merci beaucup"--many thanks. "Bookoo B.S."--overabundance of B.S.