A bit like the 'gimme' that got me in the 'magnetism' game a few days ago. Actually, that may have helped me this time, because I saw 'damme' (uncommon) first, before seeing 'dammit'.
I still think there was a ruling a while back that all these colloquial contractions (gonna, oughta etc) were to be uncommon, but can't find it in the archives.
MK