The small wet stain on your underwear (and, in desperate cases, your trousers) caused by putting your dick back after taking a piss without shaking properly. Named after the English national football stadium (presumably due to the shape of the stadium). Not something that impresses the ladies: be warned, splashes from the sink when washing hands can give the same effect. Um... but you should still wash your hands, of course. Particularly evil on light coloured trousers.
You probably don't want to say "Nice Wembley, mate!". You shouldn't be looking.