English words constantly evolve, not only in terms of what they mean, but how they mean. They transform their parts of speech all the time without so much as a changed syllable. The adjective green came to mean the part of the golf course that can be described by this adjective. The prepositions up and down came to mean the experiences in life that feel like the spatial relationships that these prepositions describe--life's ups and downs. We transform proper names willy-nilly into adjectives, such as when we see a dress our friend Jessica would love and describe it as "so Jessica." But the most fascinating method of linguistic conversion is to verbify--an autological (self-describing) word denoting the transformation of a noun into a verb. Such a process recently occurred with the noun adult, which became to adult, or, more commonly, adulting.
Verbification happens all the time. Soon after the website Google allowed us to search web pages for specific keywords and phrases, we verbed that act into Googling.We tend to verb animal names to mean human behavior that evokes that animal, such as parrot, grouse,and monkey (or horse) around. Verbs as common as to access began as nouns (we once said "to gain access"). We could probably understand almost any noun as a verb, given the right context: "The flight attendant Pepsied my cup" or "I playlisted all your song recommendations" or "Can anyone peace the world?" Calvin said it best (to Hobbes, of course) in a 1994 Bill Watterson comic strip: "Verbing weirds language."
But what allows some nouns to become widely verbed? To me, commonly verbed nouns typically seem to contain an action within them--a specific Deed linked closely enough to the Thing for the conversion to be intuitive and useful. You can only do one thing on the website Google; we all know what a parrot is famous for doing all the time; having access is inextricable from the activity of accessing. Meanwhile, Pepsi, playlists, and peace have a multitude of associated activities and therefore must rely heavily on context if we're going to verb them. To turn a Thing into an Action, you need the Thing to be bound up in an associated Action to begin with, so the meaning of the new verb unfurls naturally from the old noun.
So what action unfurls naturally from the noun adult? Adulting means more than just "reaching biological maturity" or "becoming fully grown." To adult is to engage in the responsibilities of modern adulthood. Filing taxes, cooking a meal, buying renter's insurance--Millennials coming of age use the verb to describe engaging in the mundane tasks of mature life with characteristic self-effacing irony. The so-called snowflake generation, for whom stages of development, such as starting a family or owning a home, have commonly been delayed, engages in the normal day-to-day activities of adulthood with a smirking, surreal surprise. Adult became adulting as a generation entered that period feeling somewhat unprepared, wanting to express that maturity means not only reaching a certain point in your life, but also attending to the concomitant tasks. Hence our Thursday-level clue: "Doing grown-up tasks, in Millennial slang."