It means "unexpected", because it was unexpected that a long- and well-known species was actually undescribed [since the original description was actually a Paramesotriton]. Similarly, Mantella expectata was named because it WAS expected to be described, since photos appeared years earlier in DATZ magazine of what was obviously a new and undescribed species.
Scientific names can mean anything at all...including nothing at all. Modern taxonomic descriptions normally include an etymology [word origin] for new names, so that people are not left second-guessing for centuries, and so that mistakes can be easily corrected. There are many names now which include the acronym for an agency or organization. There are also many names for which the root words aren't used for their meanings, so much as for the fact that related organisms have the same root word. Viz, Phyllobates, Dendrobates, Epipedobates, and Phobobates were named for their lifestyle: "leaf-walker", "tree-walker", "upon-the-ground-walker", and "fear-walker" [runs and hides]. Adelphobates, "brotherly-walker", however, was not named for its movement, but because it is a "brother" to a number of other frogs named "-bates"
Mistakes such as the naming of "Dendrobates dorisswansoni" and "Batrachoseps wrighti". Both of these names are grammatically wrong, and have subsequently been corrected to "Dendrobates dorisswansonae" and "Batrachoseps wrightorum". The "i" termination indicates a name honoring a man, but the frog was described to specifically honor a woman, which necessitates an "ae" termination. The salamander was described to specifically honor two people, necessitating a plural "orum" termination. It might not be possible to figure out these details unless details of exactly why a name was chosen, are actually included.
Sadly, sometimes the terminations are used in ways which are rather un-Latin, such as using a place name or habitat [ense, ensis, anus, aticum, anum, etc...] as if it's a person's name [ae, i, orum]. Other times, the endings have nothing to do with Latin grammar, and are instead what's called a "noun in apposition". That is, the word is a noun used as-is, as an adjective. We do this in many languages - the word "notebook" is a noun, but it is used as an adjective in the phrase "notebook computer". When a person's name already ends in "i", "ae", or some other inconvenient letter combination, sometimes it's just simpler to treat it as a noun in apposition, rather than play with difficult or confusing grammatical spellings.