childish or colloquial shortening of mamma, by 1823. "Also applied colloq. to a middle-aged or elderly woman, esp. one in authority" [OED].
Entries linking to ma
"mother," a word used especially by children and infants, 1570s, representing the native form of the reduplication of *ma- that is nearly universal among the Indo-European languages (Greek mamme "mother, grandmother," Latin mamma, Persian mama, Russian and Lithuanian mama "mother," German Muhme "mother's sister," French maman, Welsh mam "mother").
Probably a natural sound in baby-talk, perhaps imitative of sound made while sucking. Its late appearance in English is curious, but Middle English had mome (mid-13c.) "an aunt; an old woman," also an affectionate term of address for an older woman.
In educated usage, the stress is always on the last syllable. In terms of the recorded appearance of the variant or related words in English, mama is from 1707, mum is from 1823, mummy in this sense from 1839, mommy 1844, momma 1810, and mom 1867. Mamma's boy "soft, effeminate male" is by 1901.