Yours, Etc.: Origins and Uses of 8 Sign-Offs
before vowels mast-, word-forming element meaning "breast," from comb. form of Greek mastos "woman's breast," from madan "to be wet, to flow," from PIE *mad- "wet, moist, dripping" (cf. Latin madere "be moist;" Albanian mend "suckle;" see mast (n.2)).
masto- or mast-
Breast; mammary gland; nipple: mastectomy.