google
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early 20th century: from momma + -y2.
wiktionary
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From mom + -y.
etymonline
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mommy (n.)
1844, U.S. variant of mamma. Variant spelling mommie attested by 1882. Mommy track "career path for women that puts priority on parenting" is attested by 1987. Related: Mommies; also see momma.
