A fish apple can occur when you store an apple next to (say) mackerel sandwiches in (say) a butty box.
when you chomp on the apple, it tastes a bit fishy. This is not a good thing. Helpfully, a respberry yoghurt will remove all traces of the unpleasant fishapple flavour from the affected pie-hole.
That happened to me yesterday. I thought I'd do general chat about it, to warn others.
Interestingly, my missus once put my headphone in a bag with some fish (don't ask, she's an idiot sometimes) and I developed an curious case of fishphones, which are actually worse than you'd imagine.