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a combining form meaning “breast,” used in the formation of compound words:
Also, especially before a vowel, mast-.
Origin of masto-
combining form representing Greek mastós breast Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for masto-


combining form
indicating the breast, mammary glands, or something resembling a breast or nipple: mastodon, mastoid
Word Origin
from Greek mastos breast
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for masto-

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)).

Online Etymology Dictionary, © 2010 Douglas Harper
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masto- in Medicine

masto- or mast-
Breast; mammary gland; nipple: mastectomy.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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