It's a a slang. אחי is "my brother", also said for anyone you meet (like a seller in a store), not just your real brother, and אחויה is a variation, that you would say only to close friends, like "bro" in american-english. only young people use these both אחי and אחויה if you're interested there's a book = "the big slang dictionary" (dont know if this specific one appears there) https://simania.co.il/bookdetails.php?item_id=45169