Vocalissimus

Vocalissimus comes from the Latin adjective vocalis, meaning "that utters a voice, sounding, singing," from which English gets its word vocal. In the rare poetic use, vocalis can also mean something of a muse: "causing or inspiring speech or song." In the case of vocalissimus, however, there is some special grammatical indication; it is in the superlative, with masculine vocative case. This means that when Stevens mentions Vocalissimus in "To the Roaring Wind," it is in direct address to a male, possibly one who is acting as a muse to Stevens; and because it is superlative, it indicates that this Vocalissimus is the best at his vocalization.
==Source==
A Reader's Guide to Wallace Stevens, by Eleanor Cook, on Google Books