mamma vs knocker

mamma and knocker both are nouns.

Word	Noun	Adjective	Verb	Adverb
mamma	Yes	No	No	No
knocker	Yes	No	No	No

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

mamma: milk-secreting organ of female mammals
knocker: either of two soft fleshy milk-secreting glandular organs on the chest of a woman

Other hyponyms of mamma include dug, bag, udder, boob, bosom, breast, tit, titty.

mamma (noun)	knocker (noun)
milk-secreting organ of female mammals	a device (usually metal and ornamental) attached by a hinge to a door
informal terms for a mother	either of two soft fleshy milk-secreting glandular organs on the chest of a woman
	one who disparages or belittles the worth of something
	a person who knocks (as seeking to gain admittance)
	(Yiddish) a big shot who knows it and acts that way; a boastful immoderate person