In rDzong-kha, knife or dagger is called gyi. In Tibetan it is gri. But interestingly archaic Tibetan materials allude to ral gyi. See, for instance, Go-shul, gNa’ rtsom (p. 128, n. 10).
And what about the etymology of ral gri “sword”? (By the way, in rDong-kha, one would not say ral gyi but it has to be ral gri.) Apparently a “knife” (gri) [for] tearing/slicing [something/someone] apart (ral).”
The etymology of spu gri “razor” is self-explanatory; a “knife [for shaving] hair.”
We may also speculate with the etymologies of other kinds of knives.