Definition of "knockers"

[…] from the Rev. Mr. Cannaday, “who winks at you to show that he’s one of the boys,” to the thrice-married Norma Norbridge, “who leans on her knockers instead of her elbows when she sits at a table.”

1950, Richard Mealand, The First Person, page 116

I bet her knockers are bigger than any girl in our grade, whaddya think?

1984, Sharon Berman, With a Face Like Mine, page 35