Green’s Dictionary of Slang

(US) excellent.

1923		H.C. Witwer Fighting Blood 116: And this watch Nate give me is plenty timepiece, too.
1933		Fortune Aug. 47/1 In sum, Mr. Brown plays plenty trombone or, as his friend suggested, a gang o’ horn.
1941		R.P. Smith So it doesn’t Whistle 53: When they want to say a man’s good, they say he plays plenty sax or plenty drums, but this 'solid-sender' and 'big gate' and all that self-conscious jive talk is made up for and by the teddy-bear haircut guys .
1970		C. Major Dict. Afro-Amer. Sl. 92: Plenty, good, excellent.