For anyone wondering if real piss is involved, this is from the Wiki:

There is a misconception that century eggs are prepared by soaking eggs in horse urine, but this is not true. In Thai and Lao, the common word for century egg translates to “horse urine egg”, due to this misconception or due to the eggs’ ammonia odor:

Thai: ไข่เยี่ยวม้า [kʰàj jîaw máː] (RTGS: khai yiao ma)

Lao: ໄຂ່ຍ່ຽວມ້າ [kʰāj ɲīaw mâː]