Green’s Dictionary of Slang

(orig. Aus.) a masturbation game, popular among schoolboys, whereby the participants masturbate and then ejaculate onto a biscuit; the last to reach orgasm must eat the semen-covered biscuit; the biscuit itself.

1997		Sydney Morning Herald 13 Feb. 30: Saos have been providing entertainment for bored private boarding school boys for many years in the character-building group activity known as ‘soggy Saos’.
2000		Chickenhead Productions 🌐 You know the drill—pantless pledges gathered around, each breathlessly racing to make sure he’s not the last to spill seed, lest he be the sorry chump forced to wolf down the dreaded Soggy Biscuit.
2000		Courier-Mail (Brisbane) 22 Aug. HO2: Can you imagine me at King’s, surrounded by Maccas, wearing one of those stupid little caps, playing soggy Sao?
2002		www.soggybiscuit.co.uk 🌐 The main idea is that a group of 2 or more men get together in a circle. A biscuit is distributed among each member (digestive, rich tea, or bourbon are all good choices). Then, after the signal, they all begin masterbating [sic] with the biscuit strategically placed at the end of their penis. As time passes, they each ejeculate [sic] onto the biscuit. The last one to complete the task (and it must be completed) must eat their ‘soggy biscuit’. If the ‘loser’ is particularly unliked by the group, then they may be asked to eat everyone elses biscuits.
2016		Twitter 30 June 🌐 These public school boys eh? All this because Boris lost a round of ‘milky biscuit’.