Mean can mean you are being rude or in math terms it is adding a set of numbers then dividing by the total amount. ex. If your numbers are 1,2,and 3 add them to get 6 and divide by 3. You get 2.
It is a form of baptism in the Christian theology. Young infants can be totally immersed as the Southern Baptist do. It is also used when a Child is christened and has his name blessed,