The correct spelling for the opposite of voluntary is "involuntary." However, some sources may use "unvoluntary," which is not recognized as standard English. The pronunciation of "involuntary" is /ɪnˈvɒlənt(ə)ri/, with the stress on the second syllable. The prefix "in-" indicates negation, and the "v" sound after the "in" corresponds to the "v" sound in "voluntary." Therefore, it is phonetically logical to spell it as "involuntary" rather than "unvoluntary."
The word "unvoluntary" is derived from the combination of the prefix "un-", meaning "not", and the adjective "voluntary", which is derived from the Latin word "voluntarius". The Latin word "voluntarius" comes from "voluntas", meaning "will" or "wish". Therefore, "unvoluntary" essentially means "not intentionally or willingly done". However, it is worth noting that "involuntary" is more commonly used instead of "unvoluntary", making the latter somewhat archaic.