Inert/ɪˈnɜːrt/ Definition 1 of 3Pro Tip 1/2Physical immobility Used to describe something or someone that is not moving and seems almost lifeless. After hours of work, he was lying inert on the couch. Pro Tip 2/2Lacking energy Can imply a lack of energy or enthusiasm in a person's behavior or state. He responded to the cheer with an inert nod.