All classic rabbinic uses or similarities to this word make no sense in this context: Source. Womb. Passive version of cold?! Everything seems patently wrong. I even checked Radak's own Seffer Hashorashim to see if he gave the word any special meaning. No luck.
Throughout his various commentaries, Radak used the word מקור dozens of times in explicit reference to the Hebrew infinitive. He does not appear to have discriminated between the infinitive construct and the infinitive absolute, which differentiation only seems to have appeared centuries later in the pedagogical Classical Hebrew grammars by the 19th Century Hebraists Baer, Davidson, Ewald, and Gesenius among others. Interestingly, one of these Hebrew grammars uses the word מקור in reference to the Hebrew infinitive.