Muhmal: not assigned meaning
مُهْمَلmuhmal - unassigned meaning
Muhmal means an uttered expression that has not been assigned a recognised meaning in Arabic grammar.
Simple explanation
مُهْمَل (muhmal) is an uttered expression that has not been assigned a recognised meaning in the language. It may be pronounceable, but it is not treated as meaningful speech.
Nahw focuses on meaningful speech, so muhmal is introduced mainly to show the boundary before grammar begins.