Green’s Dictionary of Slang

(gay) ‘To insert the glans of one penis into the foreskin of another and then masturbate to mutual ejaculation’ Simes (2018).

[1881		Sins of Cities of Plain (1992) 48: My favourite idea was to pull back the skin of my foreskin, and doing the same to my bedfellow’s prick, bring the nose of his affair to mine. Then I would draw the skin of mine over the heads of both cocks, and we’d fuck each other gently so. What a deliciocus thrill we had when spending, the seed seeming to shoot backwards and forwards from one cock to the other!] [Simes:DLSS].
1988		H. Max Gay (S)language 12: docking—mutual masturbation requiring that one partner have an exceedingly long foreskin, drawing it over the head of the penis of the other—then, sharing the foreskin, mutual masturbation to mutual ejaculation.
1988		OutRage (Melbourne) Mov. 7/1: [classidied advert] Uncut guy like to meet similar, also someone wo knows stretching methods. Mine is rather tight, and I’d like it floppy for docking etc [Simes:DLSS].
1995		G. Griffin Penis Size 117: One sex act that has gained inpopularity with the rise of safe sex is ‘docking’ in which the foreskin of one man is drawn over the penis of his partner. Both penises are then masturbated simultaneously [Simes:DLSS].
1997		Pump It Up (N.J.) Spring n.p.: [personal advert] Hot, greasy. Precum uncut manmeat cock..wants other 501 Levi-bulging manmeats to pump/bloat/dock/showoff, get nasty with [Simes:DLSS].
2001		SX (Sydney) 5 Mar. 38/2: If sucking, jerking and occasional rimming and docking isn’t your scene this definitely isn’t the video for you [Simes:DLSS].