Hello Cassie "I think that 'have at' = attack. Is there any other meaning? Also, is 'have at it' a derivation of the former? Why could people use it to mean 'go ahead' or 'help yourself'?"----Cassie Yes, your understanding is exactly proper to the usage. If you are at a social gathering, and tables of food and drink are laid out for the expected guests, someone might show up quite early. They might say very coyly, "Can I sample some of the delicious food?" You might answer, in an offhand way, "Have---At--It!" Exactly. It means you do not care at all if the person does something. The two expressions you cite are basically the same. Brevity is often welcome. So, somone might just say "Have-At" and it means the same. Very quick communications, which convey great power, are common in some social contexts. This is one way we show complete empowerment in the English Language. To say "Have at it." is to tell someone that they have complete freedom to do absolutely anything they want. Here's another circumstance. Let's say I have met a nice person, and they invite me to their house. I am immediately drawn to the large shelves of books in that persons house. So, as they head off to use their bathroom, or go to the kitchen to get us something to drink, I say; "Can I look at your books?" They could answer; "Have at it!". It's a nice way to make a person feel that "My house, is your house!" Okay? Great Question! ---Warm Regards, Bruce

Have at - Attack or make an attempt.