To act up is to act in a way different from that which is normal or expected. During this profanity-laced song, the City Girls show they are more than happy to "act up" in order to take advantage of rich, dumb men. They do this by making themselves attractive to these dudes, even if they are already in a relationship.
The song's rise up the Hot 100 was aided by the the #ActUpChallenge on Instagram which consists of teens posting video clips of drinking, smoking, and twerking along with photos of exes and former friends. The internet challenge is not age-appropriate and has gotten a number of youngsters into trouble.
Lil Yachty is credited as a co-writer on the song (under his real name of Miles McCollum) along with The City Girls' Jatavia Johnson and producer EarlThePearll. Lil Yachty has worked frequently with EarlThePearll and is on Quality Control, the same record label as City Girls.
Speaking with Kerwin Frost, Yachty revealed that he personally penned the entire song (except for J.T.'s last verse) while in the studio with EarlThePearll. It was his first time writing a song for another artist, and he accomplished the task by getting into The City Girls' raunchy mindset.