Troll

The worst grade wizards or witches can get on their O.W.L's/N.E.W.T's. In highest to lowest the grades go: O - outstanding, E - exceeds expectations, A - acceptable, P - poor, D - dreadful, T - troll

Gryffindor student: What'd ya get on your defence against the dark arts O.W.L?
Hufflepuff student: I got a T for troll.
Gryffindor student: That's got to be rough.