On Primus' song Lacquer head, Les does this left handed muted slap. Basically, during the verse, he thumps through the E string, left hand mute slap once, pluck once with right hand, and left hand mute slap again, and the pattern goes on and on. Does anyone know how he does that mute slap with left hand? I tried and all I could get is a tapped G (on E string)...

I believe that it involves the fretting hand index finger muting the strings.

It's the thumb that mutes the strings at around 2nd fret on the E string, but when the fretting hand "slaps", it hits around the 3rd fret, so the G always pops out...