The phrase "to reject the breast" means that a baby or young child refuses to drink milk directly from their mother's breast during breastfeeding. Full definition
And smart as he / she already is, your little one might start to reject the breast quite rapidly if you offer the bottle too soon. (easybabylife.com)
Many breastfed babies don't accept a bottle, however, and there is also the risk of nipple confusion for babies starting to reject the breast if introducing a bottle too early. (easybabylife.com)
They may also prefer the taste of formula, causing them to reject the breast entirely. (romper.com)