It's also common as a gerund—that is, in the form adulting as a noun, as in " Adulting is hard" or "I'm not very good at adulting." To adult is to behave like an adult.
Adult definition is - fully developed and mature: grown-up. How to use adult in a sentence.
A fake word used by idiots (millennials mainly), in an attempt to turn the adjective adult into a verb in a feeble attempt to describe if a person is actually an adult.