It originated from students in the South of England who were studying Koine Greek. The word lame has been given the Greek adjective ending -os, but carries the same meaning.
An over the top misspelling of LMAO, which can be used intentionally as a substitute, this is what all the kool kids use these days.
Pronounced: LA MOSS or LA MOWSE
Person A: "my nan gotrun over by a snowmobile today"
Person B: "lamos"