Your A-card is the V-card of anal sex, or your anal virginity. For men, you can lose your A-card giving or receiving. Women can only lose it on the receiving end; pegging doesn't count.
"Dude, a bunch of girls are begging me to take their V-card. What do I do?"
"Keep your V-card for someone special, but drop your A-card to one of these random sluts if you please. Tell them the A-card and the V-card are the same thing and they'll shut it."