Plain English Meaning
It isn't working. The instrument or system is broken, failed, or out of service and cannot be relied on.
Full Definition
Not working or not functioning. In aviation, the term refers to an instrument, system, or piece of equipment that is unserviceable, failed, or otherwise not available for use during flight or ground operations.
Context Anchor
Seen in preflight checks, equipment lists, instrument failure discussions, and training for flying when one or more cockpit instruments are not usable.
Derivation
From Latin 'in-' meaning 'not' and 'operari' meaning 'to work.' Literally 'not working.' The aviation use stays close to the everyday meaning but carries specific : an inoperative item often has rules attached about whether the can legally fly with it that way.