A tempting sexual liason with someone you really shouldn't due to any of the following reasons - too old, too young, too drunk, too mental, too disabled - but still posess a certain 'Je'nais ce quoi'
Something thats 'Good, but not right' (Mr Walker's catchphrase in 'Catchphrase')
"Saw you in the corner with Laura"
"Aye, reckon I'm in there"
"You do know her dad just died"
"Woah! Roy Walker!"
"It'll be good, but not right"