knack ‧ er /ˈnækə $ -ər/ BrE AmE verb [transitive] British English spoken informal
[ Date: 1800-1900 ; Origin: knacker 'horse-killer' ; KNACKERS ' YARD ]
1 . to become extremely tired SYN exhaust
knacker yourself (out)
Slow down – you’ll knacker yourself out!
2 . knacker your elbow/hand etc to hurt your elbow etc so that you cannot use it