1. Sucking on a partner’s unwashed penis after it has been up a partner’s bum. Far more popular in porn than in real life. Given how there’s still plenty of ass germs on a penis after A2M, it’s best not put a penis into a vagina after A2M without washing it with soap and water first. 2. A protein coding gene (Alpha-2-Macroglobulin), but anyone looking for that definition of A2M is probably on the wrong website.