Dude, if you must have an etymology, have an etymology:

lame (adj.)
O.E. lama. from P.Gmc. *lamon (cf. O.N. lami, Du., O.Fris. lam, Ger. lahm "lame"), "weak-limbed," lit. "broken," from PIE base *lem- "to break" (cf. O.C.S. lomiti "to break," Lith. luomas "lame"). Sense of "socially awkward" is attested from 1942. Verb meaning "to make lame" is attested from c.1300. Related: Lamely; lameness. Lame-brain (n.) is first recorded 1929.

So yeah, the disabled sense far pre-dates anything else. But the point is, the etymology *doesn't really matter* if it's currently being used as an insult towards actual people.