In the Roundworld and Discworld this is street slang for a quickie up against a wall with a prostitute or seamstress in Discworld- a quick shag for tupence. Due to inflation, prices are likely higher in both places but the popular title still remains.
It is also a kind of jam donut or jam-based dessert served with hot custard, but whether the donut took its name from the intimate sexual service or the other way around is not known.
The first mention of the expression is in Night Watch during the events leading up to the Glorious Revolution when Rosie Palm offers a bribe when she is trying to escape the clutches of the Cable Street Particulars.
A 2p Ankh-Morpork Post Office stamp honors the Seamstresses' Guild.
The euphemism "seamstress" was used for a prostitute in Roundworld as well as in Discworld. It is often found on old census forms and is obvious when the form shows a large number of young women living at the same address. Seamstress was a profession that young women often followed and the pay was too poor to survive on, so it is not surprising that they often ended up in the "other" seamstress line of work.