"Hath at it" is a phrase that can be used to mean "begin" or "start", especially in regards to a fight or competition. There are several synonyms that can be used in place of "hath at it", such as "commence", "initiate", "launch", "kick off", and "embark upon". Other similar phrases include "let the games begin", "get started", and "take the field". These phrases are commonly used in sports and other competitive settings, but can also be applied to other situations where action needs to be taken to start a task or project.