He swung down sharply, nearly catching himself a nasty crack on the jaw
with the doorstep and tumbled through the air, so suddenly stunned with
what a profoundly stupid thing he had just done that he completely
forgot the bit about hitting the ground and didn’t.
(The Ultimate Hitchhiker's Guide to the Galaxy; Douglas Adams)
Does the part in bold mean that 'he nearly hit his jaw against the doorstep, which, presumably, would crack the jaw in a nasty way'?
It does mean that he nearly hit his jaw on the doorstep, but the crack might just be the sound (and/or feel), not necessarily an actual fracture of the jaw. 'Nasyt' here just means 'very unpleasant'.