A macca is where you insert your fingers (you can put 1 finger to your whole fist inside) into your bum hole. It is also known as fingering but inside of your bum instead of your vagina.
When someone chats macca, they are speaking utter nonsense and are full of shit.
Olyf: This lass came into the club the other day, straight 10. She looked about 24 and she came up to the bar and added me on Discord.
Jacke: You are chatting pure macca, Olyf. What a buffoon-like thing to say.