Green’s Dictionary of Slang

(US) a close friend with whom one pursues most of one’s daily activities.

1902		Ade Girl Proposition 144: He sent word to some of his former Running Mates to meet him at the Train, as he could transact his Business in about 20 Minutes, after which he would remove his Bridle and begin to burn Holes in the Track.
1906		Spokane Press (WA) 9 Feb. 3/1: O’Keefe and his running partner were going along the road when reilly stopped them.
1922		N.-Y. Trib. 16 July 6/3: Reina is a working girl [...] at Woolworths [...] Reina’s running mate, Lula, [is] right across the aisle.
1947		M.H. Boulware Jive and Sl. n.p.: Rhode ... Running mate.
1961		C. Himes Pinktoes (1989) 16: He wants to know do I know Ez. Me and Ez is running buddies.
1965		C. Brown Manchild in the Promised Land (1969) 99: I didn’t hang out with any of my old running partners.
1970		G. Scott-Heron Vulture (1996) 27: My running men during the corner years [...] were always at least two years my senior.
1972		Milner & Milner Black Players 18: He loved the flash, style and humor of the Black players and valued them highly as ‘running partners.’.
1974		V.E. Smith Jones Men 150: The dope wants to know why I ain’t doing with my running buddy.
1980		E. Folb Runnin’ Down Some Lines 24: Many high-school age youngsters had ‘running partners’ or play brothers and sisters.
1988		D. Woodrell Muscle for the Wing 69: I heard you and him was runnin’ mates back there in yesteryear.
1997		Simon & Burns Corner (1998) 9: Gary grins beatifically at his running buddy.
1999		E. Bunker Mr Blue 180: I had several black friends [...] but they were not running partners. I did not walk the yard with them.
2000		C. Cook Robbers (2001) 4: Them talking that way because they were running buddies.
2000		S. Maloney Big Ask 240: Roscoe and Len, Donny’s putative running mates, turned away shamefaced when I looked their way.
2004		Simon & Lehane ‘Dead Soldiers’ Wire ser. 3 ep. 3 [TV script] My running buddy don’t even tell me where he’s at, these days.
2008		R. Price Lush Life 226: How about a running buddy. Somebody he likes to jux heads with out there .
2021		T. Swerdlow Straight Dope [ebook] Was it her boyfriend, a dealer, a running partner?