Maybe from Latin verbum (“word”) through *vărbu, subsequently *vărbă by singularization of the supplanted neuter plural up to vorbă in analogy to the change from to in the imperative vino from vină. The intermediate form *vărbu is strengthened by related Aromanian vãrghescu (“to quarrel, scold, urge”), having turned into which is a common trait. For instance, in the singular corbu and in the plural corghi.
Conversely, it might have come from Old Church Slavonic дворьба (dvorĭba), from *dvorъ (“court”), as sustained by the presupposed variant horbă (“gathering, word”). In the 17th century it still had the meaning of "gathering", attested by Dosoftei, but already by 1744, as attested by Ion Neculce, it seems to have shifted to "word" in case this meaning was not already encompassed previously. Moreover, in old texts, a word dvorbă, presumably of this etymology, meant "service to the court", and was never confused with vorbă.
vorbă f (plural vorbe)