Well, believe it or not: It’s crocodile dung.
It all began in Egypt where this very first contraceptive was created. Amorous Ancient Egyptian women used crocodile pooh pessaries as contraceptive. Documents dating back to as far as 1850 BC refer to this method of contraception.
Crocodile dung is actually slightly alkaline, like modern-day spermicides, so it could have worked. On the other hand, it may well have been that after application neither party felt much like making love.