Green’s Dictionary of Slang

knacker n.

[SE knacker, a horse-slaughterer]

1. a worn-out horse, fit only for slaughter.

[UK]Mr Mathews’ Comic Annual 18: I say, aggravating Sam, vot’s the worth o’ your two knackers. Vy, that von’s vorth fifteen shillings alive, and von pound five vhen dead.
[UK]Flash Dict. in Sinks of London Laid Open.
[UK]Hotten Dict. of Modern Sl. etc.
[UK]J.A. Hardwick ‘The Browns Ruralising’ in Prince of Wales’ Own Song Book 41: He owned a knacker – thorough-bred [...] With bony limbs and skinny head.
[UK]Illus. Police News 30 Mar. 3/3: He had a knacker belonging to ‘Coper Teddy’ to sell.
[Aus]Dead Bird (Sydney) 20 July 2/4: A raw-boned goose-rumped knacker.
[Aus]Dead Bird (Sydney) 22 Mar. 7/2: A pony which can donkey lick any 14 2 knacker in N.S.W.
[Aus]Truth (Perth) 25 Feb. 8/8: But me cab and quiet knacker / As i do drive of a day.

2. by ext. a worn-out, useless person.

[UK]H. Nisbet Bushranger’s Sweetheart 64: He ought to be sold and salted for being such a slow old knacker.

3. (Aus.) a A$2 bill.

[Aus] ‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxv 6/3: knacker: A two-dollar bill. A pound was known as a knicker and the ram showing on the two-dollar suggested the knacker.

4. (Irish) a general insult aimed at a person.

[Ire]R. Doyle Snapper 46: Don’t think you can stroll in and out of here when you feel like it and shout language like a – like a knacker.
[Ire]P. Howard PS, I Scored the Bridesmaids 205: Boggers everywhere, in their county colours, like the knackers they are.

5. (Irish) a traveller, a tinker.

[Ire]P. Howard The Joy (2015) [ebook] [They] started exchanging a bit of rapid-fire patter, which me and Redser didn’t understand. It’s like they have their own bleedin language or something, those knackers. [Ibid.] ‘I’ve nothing against knackers, right? Live and let live, that’s my fuggin motto’.

6. (UK juv.) a thief.

[Ire]G. Coughlan Everyday Eng. and Sl. [Internet] Gouger (As used by Dublin Gardaí) (n): a dangerous knacker/thief.
OnLine Dict. of Playground Sl. [Internet] knacker (1) n. Unwashed thieving person, often said to be found living in caravans on laybys on B-roads in Britain.