(for pure speculation and reflection)
I think, rather, that gzugs is related to zug pa, "afflict" and that the word is derived from the Sanskrit etymology of rūpa as something that has the essential characteristic of being able to be broken/damaged/afflicted (√rup). BTW, excellent website!
Dear Ratna,Thanks for your insightful comment/suggestion. It is, of course, clear that not just in the case of rūpa (gzugs) but in several other cases, Tibetan translators indeed often tried to consider the Sanskrit etymology when translating a Sanskrit word into Tibetan. I am aware of the gzugs su rung ba etymology of gzugs, that is, “something that is gzugs-able,” but even after accepting that etymology, one would still like to ask what gzugs is. I also assume that there is an etymological link between zug (or zug rngu “pain”) and gzugs. Possibly the noun gzugs is secondary and gzugs pa (or the form ’dzug pa) as verb is primary, which means “to prick, to stick into” or “to thrust, to pierce” (Jäschke 1881: s.v. ’dzugs pa). Therefore, etymologically a physical entity with which one can “pierce” or “sting” (something or someone) or “to cause pain” (or as you say to “afflict”) is a gzugs. By that very token, only those physical objects that can “pierce” can also “stand erect.” Likewise gzugs po (as verbal adjective) means “to be erect or upright or stiff,” and adverbially also gzugs por (“in a straightforward manner”). In short, I do not see a contradiction between our propositions.Thank you once again, and please do keep commenting.With best wishes,Dorji