Green’s Dictionary of Slang

grogans n.

[generic Irish family name Grogan; such whiskers were popular among Irish-Americans]

(US) muttonchop sidewhiskers.

[US]T.A. Dorgan in Zwilling TAD Lex. (1993) 107: (How Some Fighters Would Look If They Followed Jeff’s Whisker Fad) Jim Corbett with ‘Grogans’.
[US] cited in Zwilling TAD Lex. (1993).