An unexpected erection resulting from the insertion of a plastic tube (or, in the case of The Cider House Rules, a bamboo shoot) into the urethra, aka "pee hole."
Nurse: This may feel a bit uncomfortable going in.
(A moment of silence passes.)
Fellow: I have an erection.
Nurse: Oh, yes, catheterwood. That happens sometimes.
Fellow: I feel...proud.
Nurse: I can tell. At least you don't have any accessory catheternips.