On a doughnut or
donut, the
jam anus is the insertion point of
Jam into a filled doughnut.
This is conducted using a baster or piping bag, thus a puncture mark is left on the doughnut when the implement is retracted.
The term was used by Sarah Millican in the Off Menu Podcast with Ed Gamble and James Acaster.
Episode 87: Sarah Millican (Christmas Special) whilst discussing Greggs doughnuts (29:11)