Ain’t Jenny is coming to town
Autocorrect usually works by changing a letter in a word when you input a typo. However, there are so many words in the English language that have similar spellings that autocorrect might have difficulty determining which word to use. In this case, it couldn’t tell whether U or I belongs in Aunt.

Obviously, it chose the letter that made the word make absolutely no sense. Ain’t Jenny? Even Microsoft Word doesn’t recognize this word, and we know how “reliable” it is at spellchecking. What surprised your mummy wasn’t that you couldn’t spell Aunt but that you didn’t bother to fix it in the four hours it took for her to respond.