wrecker vs knacker

wrecker and knacker both are nouns.

Word	Noun	Adjective	Verb	Adverb
wrecker	Yes	No	No	No
knacker	Yes	No	No	No

As nouns, knacker is a hyponym of wrecker; that is, knacker is a word with a more specific, narrower meaning than wrecker:

wrecker: someone who demolishes or dismantles buildings as a job
knacker: someone who buys old buildings or ships and breaks them up to recover the materials in them

Other hyponyms of wrecker include housebreaker, housewrecker.

wrecker (noun)	knacker (noun)
a truck equipped to hoist and pull wrecked cars (or to remove cars from no-parking zones)	someone who buys up old horses for slaughter
someone who commits sabotage or deliberately causes wrecks	someone who buys old buildings or ships and breaks them up to recover the materials in them
someone who demolishes or dismantles buildings as a job