Green’s Dictionary of Slang

very (much), extremely, used as a general intensifier.

[1892		Yorks. Post 28 Jan. 8/8: [advert] Fumez-Vous Beaucoup? Then smoke Ogden’s ‘Fruit and Honey’ pure Virginia].
1918		Arizona Republican (Phoenix, AZ) 20 Dec. 8/4: First stop in thirty days, beaucoup fatigue, billeted in chateaux.
1924		(con. WWI) A.G. Pretty Gloss. of Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: bookoo. A lot, much – from the French ‘Beaucoup.’ [Ibid.] buckoo. Much.
1932		Folkestone, Hythe [etc] Herald 7 May 4/2: In the first picture there is beaucoup, beaucoup, beaucoup green.
1936		Mencken Amer. Lang. (4th edn) 573: There was some fashioning of counter-words and phrases from French materials, e.g., boocoo or boocoop (beaucoup), toot sweet (tout de suite) and trez beans (tres bien), but neither class was numerous.
1967		P. Thomas Down These Mean Streets (1970) 102: You was in there a beau-coup long-ass time.
1968		in Edelman letter Dear America (1985) 49: Number 1 means real good and Number 10 means real bad in pidgin Vietnamese-English. Other handy phrases [...] are: titi – very little; boo koo (a bastardization of beaucoup) – very much.
2001		(con. 1964–8) J. Ellroy Cold Six Thousand 604: Pete got goose bumps. Pete hinked out. Pete hinked out boocoo.
2001		(con. 1975–6) E. Little Steel Toes 160: Faggots and dykes, and beaucoup little rock-and-roll broads vamping.