Etymologically, the Tibetan word for “etcetera” seems to mean “added to” or “accumulated on” or “heaped upon,” that is, “x la sogs pa” means “heaped upon x.” See also the dGe chos ’gsung ’bum (vol. 5, p. 80): de la sogs pa = de la gsog pa bsdus pa. Obviously meant as de la gsog or de la bsdus pa (“gathered therein” or “collected therein.” Izumi Miyazaki points out the possibility of “x las sogs pa” (with ablative), as found in archaic materials, in which case it would mean “accumulating from x.” See the brDa dkrol gser gyi me long (s.vv. la rtsogs pa & las stsogs pa).
For a long discussion, see Tshe-tan-zhabs-drung, Thon mi’i zhal lung (pp. 154–158).