Q: Here’s my question: “interior” and “exterior” are antonyms with the same ending; so are “superior” and “inferior.” Is there a matching antonym for “ulterior”?
A: An antonym for “ulterior” with the same suffix? As far as I can make out, there isn’t one.
The Oxford English Dictionary says the English adjective “ulterior,” first recorded in 1646, comes from the identical Latin ulterior, meaning “further, more distant.” The Latin root is ulter, or “that is beyond.”
The first English meaning of “ulterior,” according to the OED, was “lying beyond that which is immediate or present; coming at a subsequent point or stage; further, future.”
But in the mid-1700s the word gained another meaning: “lying beyond what is openly stated, avowed, or evident; intentionally kept in the background or concealed.”
Like the other words you mention, “ulterior” is a combination of a stem (in this case the Latin ulter) plus the suffix ior, which was used in Latin to form comparatives. The Latin ending has given us such English pairs as “inferior/superior,” “junior/senior,” and “interior/exterior.”
Although “ulterior” was formed in exactly the same way, there’s no opposite side to this coin.
But let’s play Invent a Word! If there WERE to be an etymologically similar antonym, it could be “propior,” “proximior,” “propinquior,” or the like.
In Latin, prope (an adverb) and propinquus (an adjective) mean near or close to. The Romans’ comparative adjective (nearer) was propior and their superlative (nearest) was proximus, from proximare, the verb meaning to draw near or approach.
But even if we had concocted one of these inventions as an English word meaning the opposite of “ulterior,” it probably wouldn’t have evolved the way “ulterior” has. In other words, it’s not likely that it would now mean open and aboveboard.
Oh well. You can’t have everything.