The term originally was used in secular textual criticism (the mss of the Greek philosophers, poets, etc.) to mean a text with copyist errors. It didn't have to be an error affecting meaning. It may have been something as simple as omitting a "the," which might not have any affect on the translation, but in the terminology of secular textual criticism this was "corruption of the text."
NT textual critics took up the term, notably Westcott and Hort, who used it over and over in their two volume edition of the Greek NT which introduced their theories to the world. When the KJVO movement arose, it adopted the term too, but only used it to describe Greek NTs or English Bibles that were not just the same as the TR-KJV tradition.
Personally I hate the term. I shudder at the very idea of calling a Bible "corrupt" even if it doesn't have some verses or words I think ought to be in there, or if it's a poor translation. You can't corrupt something that God inspired. It is eternal.