mary poppins, n. — Green’s Dictionary of Slang

mary poppins n.

[joc. use of film title Mary Poppins (1964)]

(US) the female breasts.

1965		T. Wolfe Kandy-Kolored Tangerine-Flake Streamline Baby (1966) 50: The letters are big, and her little mary poppins tremble honestly.
1997		Solar Project ‘Zeitgest’ 🎵 on ...in Time [album] You wanna feel my Bethlehem steel / Mary Poppins, TNT, Bristol City / I bite into your cats and kitties.