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.
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.