A person with whom you 'could' have sex but don't want to. i.e it is physically possible.
One stage lower than a 'would', two stages lower than a 'good' but one higher than a 'dud'.
John - "What is the Good, Would, Could, Dud status of your new class mate?
Dave - "2 goods, 4 woulds, 3 coulds and 2 duds. Not bad really"
Rev Tolstoy - "How is the new choir leader Deacon Johnson?"
Deacon Johnson - "Well I could I suppose but I'd have to be fucking pissed"