Green’s Dictionary of Slang

(orig. US) exceedingly.

1957		(con. 1943) A. Myrer Big War 83: I like it here. But mucho mucho. I always have.
1960		H.S. Thompson letter 26 Jan. in Proud Highway (1997) 207: Thanks mucho – or much thanks.
1971		R.D. Pharr S.R.O. (1998) 214: ‘I’m sick. Mucho sick’.
1993		I. Welsh Trainspotting 64: The trainee manager who welcomed us wis a mucho spotty punter in a sharp suit.
2001		(con. 1964–8) J. Ellroy Cold Six Thousand 201: It [i.e. a limousine] cut west. It stopped mucho quick.