Green’s Dictionary of Slang

custard n.

1. in pl., pimples, acne [the yellow pus such eructations contain].

1941		Baker Popular Dict. Aus. Sl.
1983		Tharunka (Kensington, NSW) 6 Sept. 26/2: Acne Problem: I've got a problem. I’ve got blackheas [sic], custard tops, splotches, zits, craters, pimples, ackers, dermatology, pock marks, and acne — What is there for me to do?

2. semen.

1955		‘No. 35’ Argot in G. Simes DAUS (1993).
2005		Roger’s Profanisaurus in Viz Apr. 48: lightning in a bottle n. A photographic art study which captures the moment a gentleman expels his custard arcing majestically towards the face of a beautiful young lady.
2009		R. Milward Ten Storey Love Song 57: Bobby gets his own back on the Frenchman by suffocating him with white custard, then he screws him up in the bin.

In compounds

custard pie (n.)

(US teen) the female genitals.

1935		B. Moss ‘You Got to Give Me Some of It’ 🎵 I’m crazy ‘bout your custard pie / You got to give me some of it [...] / Before you give it all away.
2001		OnLine Dict. of Playground Sl. 🌐 custard pie n. vulg. clitoris. Used as ‘I would sure like a piece of her custard pie, I would like to chew on a piece of her custard pie.’ So named because of the way the lips come together, not to mention the creamy filling that is issued.

In phrases

clear the custard (v.)

to masturbate, after a long period of continence.

1997

Roger’s Profanisaurus in Viz 87 Dec. n.p.: clear the custard euph. To have a long overdue wank. e.g. ‘Ooooooh I’ve got balls like boulders. I’d better nip upstairs and clear the custard’.