There's also 'us', as in bogus/bogosity.
Urban Dictionary: bogosity
But I think we can see the type of endings that lend themselves to 'osity' nouns.
Thank you all.
Originally Posted by BobK
I think it's possible that it's a not-very-literate back-formation from the new meaning of lame - 'not very good'. But as 5jj says, it's anyone's guess.
The definition the OP uses comes from UrbanDictionary, so I would guess you're right about the meaning:
Urban Dictionary: lamosity