/ ˈnækə(r); NAmE / verb [ vn ] ( BrE , slang )
1.
to make sb very tired
SYN exhaust
2.
to injure sb or damage sth
► knacker·ing adjective [ not usually before noun ] ( BrE , informal ):
I don't do aerobics any more—it's too knackering.
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WORD ORIGIN
late 16th cent. (originally denoting a harness-maker, then a slaughterer of horses): possibly from obsolete knack trinket . The word also had the sense old worn-out horse (late 18th cent.). It is unclear whether the verb represents a figurative use of “slaughter”, or of “castrate”.