Green’s Dictionary of Slang

1. (US black) a homosexual.

1960

P. Oliver Blues Fell this Morning 113: A homosexual is a ‘freak’, a ‘mellow’, a ‘sissy’, or a ‘drag’.

2. (US black/teen) a favourite boy- or girlfriend, a good friend of either sex.

1947		Baltimore Sun 22 June Magazine 6/5: Mellow man. . . see Dream dust [i.e. a desirable boy or girl].
1964		‘Return of Honky-Tonk Bud’ in D. Wepman et al. Life (1976) 68: Tell my mellows / I’ll spring ’em, ’cause I’ve got the price.
1973		D. Goines Street Players 204: I’m glad you’re lookin’ out for me, mellow.
1979		Sugar Hill Gang ‘Rapper’s Delight’ 🎵 Master G, my mellow.
1980		E. Folb Runnin’ Down Some Lines 246: mellow n. 1. Good friend. 2. Favorite lover, girl/boyfriend.
1987		D. Hebdige Cut ’n’ Mix 137: He would shout phrases like ‘Rock on my mellow! This is the joint!’.
2002		‘Touré’ Portable Promised Land (ms.) 160: We Words (My Favorite Things) [...] He’s official. That’s whoa. My mellow.

3. (US) a state of calm relaxation.

1971		Current Sl. V:4 16: Mellow, n. That part of a drug trip when one starts piecing things together or when everything seems clear.
2005		N. Gaiman Anansi Boys 83: Brother of mine, you are harshing a potential mellow here.
2012		D. O’Donnell Locked Ward (2013) 138: It was all an Establishment plot by the pigs to hassle him and do the dirty on all heads and hippies by constantly harshing their mellow.