Adulting is the assumption of tasks, responsibilities and behaviors traditionally associated with normal grown-up life, along with the implication that the individual in question does not particularly identify as an adult and that acting as one does not come naturally. Adult, in this context, is a verbed noun and is also used to describe forcing someone else, such as an employee, to behave like an adult.
Adulting is commonly associated with Millennials, who often use the term on Facebook and Twitter when posting about their activities. According to Jane Solomon, the usage of the term is related to the relatively later age that millennials are taking on conventional markers of adult life, such as settling into permanent jobs, buying homes and having children. Solomon is co-author (with Ben Zimmer and Charles Carlson) of an article in the journal American Speech discussing adulting and other new terms, "Among the New Words." Posts on Twitter with the #adulting hashtag are often about things like using a conventional oven, paying bills on time and wearing pants.
Differences between millennials and earlier generations at work are often a result of the environment in which they grew up. Job hopping, for example, is often seen as immature behavior. However, in the past, jobs were much more stable and employees who demonstrated loyalty to a company were likely to be rewarded with job security.
Donald Shandler, author of the book “Motivating the Millennial Knowledge Worker,” attributes the generation’s reluctance to commit to a single company to the experiences of seeing parents and families go through downsizing and layoffs. In that context, loyalty to employers is immaterial; flexibility and a willingness to change jobs are simply adaptive behavior.
Broadly speaking, millennials are also more likely to expect workplace concessions to work-life balance, such as reasonable hours, the ability to work from various locations and results-only expectations. Seen from that perspective, behaviors that might be tagged as "adulting" -- such as working overtime, being on-premises despite illness or emergencies and giving up vacation time -- could be considered workplace problems rather than laudable behavior.
The American Dialect Society traces this use of adulting back to 2008, from this tweet:
Grew up in a town of 2k and adulted 10 years NYC. Same values: Keeping the job. Feeding the family. Educating the kids. Buying the stuff. (@unholytwerp)