An Etymology of the Tibetan Word for Cave

In my mother tongue (i.e. Tshangs-lha), the word for “to open” is phegs as in “to open the door” (sko phegs). But because there is no system of writing, one cannot always know how it would be spelled. But in this case, I think a probable spelling would be phegs. Perhaps one could assign four different forms to it: ’pheg (future), pheg (present), phegs (past), and phegs (imperative). I speculate that this is very close to Tibetan phigs as in phigs par dka’ ba (Sanskrit: durbheda), which means “difficult to pierce/penetrate” both literally (physically) and figuratively (cognitively). One might object that Tshangs-lha phegs means “to open” and Tibetan phigs means “to pierce/penetrate.” So how can the two be similar? The Tibetan phigs in the sense of “to pierce/penetrate” also seems to have the connotation of “to split open.” The Tibetan phigs also seems to be cognate with ’bigs/’big (present). The future form being dbug, past phug, and imperative phug. It also seems to be reasonable to assume that the Tibetan word for “cave” (phug) is derived from the verb phug. A “cave” is “a pit/hole/hollow/grotto that is dug” (in rocks, cliffs, ground, etc.). Perhaps the verb “to dig” (e.g. a hole) in Tshang-lha may be spelled as ’byegs (somewhat analogues to Tibetan ’bigs).