knacker:
(also ‘knackerman‘; plural: knackers‘; pronunciation: the first ‘k’ is silent — for the right pronunciation of this word, please click here)

knackers at work at a knacker yard
a person who kills horses (and other farm animals) that are no longer fit for work, about to die, and even dead, and sells their various body parts as animal food;
a person who buys and breaks up old buildings, ships, etc., in order to sell the materials in them that are still of use;
(in Irish English, and as an insult) an indecent/ill-mannered fellow; a traveller (like a gypsy),

a knacker at an aeroplane knacker’s yard/junk yard/scrap yard
They say Tom’s grandpa was a knacker and he used to have several old and sick horses on his yard where he would slaughter them.
It’s funny how things change over time, for instance, in the past knackers used to pay the farmers for their dead animals, but now the knackers pay the farmers!
The difference between a butcher at a slaughterhouse and a knacker at a knacker’s yard is that a butcher kills animals at a slaughterhouse to sell the meat of those animals to people to eat it as food, but a knacker kills animals, mostly horses, at a knacker’s yard because they have become old and/or useless and sells the meat and other parts of these animals to the manufacturers of pet food and/or synthetic material, such as glue.
Note 2: ‘Knacker’, always with a capital ‘K’, is often found in the names of people, e.g. Johann Knacker, Barbara Knacker, Mathaus Knacker, etc. For an article that tries to explain the origin of this word, please click here.
