Knacker/ˈnækər/ Definition 1 of 3Pro Tip 1/3Colloquial Use Used informally among friends or in casual conversation, not in formal speech or writing. I'm knackered after the gym session. Pro Tip 2/3British Slang Predominantly a British term, might confuse speakers of other English varieties. Marathon training has me absolutely knackered! Pro Tip 3/3Physical Exhaustion Often refers to being physically exhausted rather than emotionally or mentally tired. After moving all those boxes, I was knackered.