knacker vs wrecker

knacker and wrecker both are nouns.

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

As nouns, wrecker is a hypernym of knacker; that is, wrecker is a word with a broader meaning than knacker:

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

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