One interpretation links it to Old French terminology 'nutrie,' which referred to the act of feeding or nurturing, particularly the mouth, or even sustenance. This etymological theory seems to hint that 'Nutt'n'Her Mouth' might, in essence, describe something related to eating or feeding. Some linguists point out that in the old days, this phrase may have even had connections to certain folkloric expressions.
It is also thought to have evolved from earlier, obscure terms in old English, which may have referred to the physical act of 'nurturing,' 'nourishing' or providing something. However, the exact origin and meaning are far from certain. nutinhermouth
Consider the components. "Nut" is slang, of course, but also a foodstuff, a piece of a tree, a fastener. "In her mouth" suggests intimacy, silence, trust, or violation. Combined, the phrase oscillates between the absurd and the erotic. Is the user referring to a squirrel storing provisions? A dental emergency? Or is it simply a teenager giggling at their own audacity? One interpretation links it to Old French terminology