Vigorously pursuing an activity, especially a fight, but also sex or some other activity. For example, Whenever they play bridge they really go at it (fight), or The new job keeps Tom at it day and night (works hard), or In the spring the dogs are always at it (sex). Shakespeare used this seemingly modern idiom for "fighting" in Troilus and Cressida (5:3): "They are at it, hark!" [Late 1500s]
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.