It is a common phrase that means a very specific thing. Your insistence to misuse this phrase, because you think the separate meanings of the words make it mean something else, shows a level of ignorance regarding semantics combined with a level of arrogance due placing your own interpretation above that of all others.

Additionally, the definitions that you found do not supply context and connotations. Indefinitely is used in the context of time with strong connotations of absolutes (as in something that goes on indefinitely will last forever/a every long time), whereas undefined is used in the area of concepts/definitions, or distance (when it's so far away you can't properly see it yet), and carries the meaning/connotation of vague much more strongly.

Thirdly, there is no such thing as a pure meaning of a word. A word is a human construct and means what we agree on. Look at the changes in the meaning of words like awful and terrific. Or do you insist on using awful only to mean that it inspires awe in you, and that something terrific is very scary?