Green’s Dictionary of Slang

1. (US) an Italian, also attrib.

1989		A. Vachss Hard Candy (1990) 127: That guido driving your car, he comes out with a piece and the Jews make him into chopped liver.
1997		(con. 1970s) G. Pelecanos King Suckerman (1998) 21: He was a guido, and he did love his pasta.
2012		C. Rosmus ‘Hangdog’ in C. Rhatigan and N. Bird (eds) Pulp Ink 2 [ebook] —Hey, Guidette,’ he yelled to her.
2013		B. Leopold ‘With One Stone’ in ThugLit Apr. [ebook] You have to convince every Guido in the room that you’re pleading for your life.
2017		T. Pluck Bad Boy Boogie [ebook] Guiido toughs gathered to watch a grudge match.

2. (US campus) someone acting in an ostentatiously masculine (or feminine) manner.

1988		Eble Campus Sl. Mar. 5: guido – male attempting to be macho, usually sporting a moustache and a gold chain.
1996		Eble Sl. and Sociability 78: In four instances, a foreign proper name is used [...] guido means ‘macho’.
2001		K. Sampson Outlaws (ms.) 7: He does always look sound though, Ratter to be fair— always did. Proper Guido, he is.
2003		Mad mag. Mar. 35: [They] invite a horde of unseemly guidos to do bodyshots out of their navels.

3. a hairstyle, a mullet n.²

1999

Indep. Rev. 17 Nov. 1: In America, the cut had been known for much of the Eighties as ‘the Guido’, ‘the shlong’, and ‘the neck blanket’.