Origin: Latin suffix -able
Unavailable has 3 different meanings across 1 category:
A communist who operated underground and was therefore not available for a political leadership position.
"The former dissident remained unavailable to take the chair because he had spent decades operating as an underground communist rather than holding public office."
In plain English: An unavailable person is someone who cannot be reached or found at that moment.
"The availability of fresh produce was unavailable during the winter storm."
Usage: This entry describes an archaic, specialized meaning of unavailable as a noun referring to a specific type of communist operative; do not use this definition in modern contexts where the word functions only as an adjective or adverb indicating unavailability. Standard usage requires avoiding this obsolete sense entirely and instead employing terms like "underground agent" for similar concepts.
not available or accessible or at hand
"fresh milk was unavailable during the emergency"
"his secretary said he was unavailable for comment"
Unavailing; ineffective.
"His attempts to reason with her were completely unavailable, as she refused to listen to anything he said."
The word unavailable comes from adding the prefix un- to available, creating a meaning that is simply the opposite of being able to be used or obtained. It entered English as a direct combination of these two existing parts without any shift in core definition.