Has at it is an informal phrase that means to engage in something vigorously or energetically. The antonyms for this phrase would include phrases that indicate a lack of energy or enthusiasm. These antonyms would include phrases such as "half-hearted attempt," "lackluster effort," "low energy," "passive approach," or "timid action." These phrases all indicate a reluctance to engage fully in a task or activity, which is the opposite of what "has at it" implies. When someone has at it, they are fully committed and energetically engaged; when they use these antonyms, they are expressing the opposite sentiment.