Often doubtfully explained as mash until no good, or a self-referencing (recursive) acronym, mung until no good. Rumored to have originated from one of the Massachusetts Institute of Technology computer groups in the 1970s or 1980s. Potentially related to mangle.
Meanings
(informal) To make repeated changes to a file or data which individually may be reversible, yet which ultimately result in an unintentional irreversible destruction of large portions of the original data.
(broadly, informal) To harm, to damage; to destroy.