Verbum mentis is, of course, a Latin expression. It stands for a mental or internal word. In the middle ages, people believed that prior to and regardless of their public (social) meaning, words exist in our minds. Thus – mental (internal) words.
The mental words are prelinguistic or extralinguistic concepts.
If we are to believe in the existence of the verbum mentis, one ought to derive the ratiocination that thinking is in a sense independent from language and symbolization. Which is something that only a few contemporary semioticians would accept.