Green’s Dictionary of Slang

lather n.1

semen.

[UK] ‘The Lady’s Snatchbox’ in Cuckold’s Nest 26: The barber, so randy and gay, / With lather he always me stocks, / And brings his long pole every day, / To stick in my little snatch box.
[UK] ‘Said A Quim To A Jock’ in Rambler’s Flash Songster 47: Now this quim was a maid, / And the jock was a blade, / Who was up to most things in nature, / To his word he did stand, / With lather in hand, / By this beautiful conquering creature.
[US]Maledicta IV:2 (Winter) 196: There are many terms for the ejaculate […] fetch, fuck, melted butter, honey, spunk and lather are all fairly common terms.

In compounds

lather-box (n.)

the scrotum.

[UK]‘Randy Mots of London’ in Libertine’s Songster in Spedding & Watt (eds) I 139: With pleasure at his pole they stare, / His lather-box and all his ware.