1
general::
Attack; also, make an attempt at. For example, Urging the dog on, he said, "Go on, Rover, have at him," or It's time to have at straightening out these files. [Late 1300s]
American Heritage Idioms
2
general::
Phrase(s): have at something
Go to get at something., Phrase(s): have at someone
to go at someone; to attack someone. • The boys had at the gang members and gave them a beating. • I just knew John was going to have at Fred.
McGrawhill's American Idioms And Phrasal Verbs