Both Jäschke (s.vv. khyi & khyi gu) and the Tshig mdzod chen mo (s.v.), which unmistakably explains khyi gu “puppy” or as “a canine youngling” (khyi’i phru gu). In addition, Jäschke has also “bud (of leaves and branches, not of blossoms)” and “eye (of a plant).” Dan, however, thinks that, in one of his instances, khyi gu means “phlegm” (i.e. the obstructions in the breathing passages that older people especially have to cough up): rtog pas rtog pa spong ba khyi gus khyi gu ’byin pa lta bu (Zhi-byed Coll. II 474.5). Honestly I do not know of khyi gu in the sense of “phlegm” (but this is not a “Docta ignorantia” and hence inconsequential).
As Dan has suggested, one would assume that khyi gu is derived from khyig gu, which in turn is connected with the verb ’khyig pa “to fasten” (’ching ba) or also “to strangle” or “suffocate.” If so, we would anticipate that khyi gu should mean something like “sling” or “snare.” To be noted is that we have several similar words such as thag/tha gu meaning “rope.” But is khyig/khyi gu in the sense of “sling” or “lasso” attested? Not that I know but we shall have to see. How is “phlegm” connected with khyi gu? Supposing that khyi gu is derived from ’khyig pa, a possible explanation would be that khyi gu means something “that strangulates/strangles/chokes/clogs.” In such a case, ’khyig pa itself should mean something like “to fasten/strangle/block/choke.” So the pertinent idea in the Zhi-byed writings seems to be that if one is choking with a ball of rice, one should eat a ball of rice to remove the blockage.