"Slam" Stewart? Milt Hinton? Upright bass slappers from the '30s/'40s. If you wanna workout, try slapping one of Mule's URB figures on yer EB. Bass Player transcribed a couple a few years back(IIRC, Darryl Jones & Hinton grace the cover). Electric bass? I guess Larry Graham is considered the source...he probably began this method back in the early '60s? I have also read a Chuck Rainey interview where he mentioned Monk Montgomery as the 1st to slap ('50s?).