Meconium is the very first poop that a baby makes after being born. It is thick, dark green, and sticky because it contains things that the baby's body had been getting rid of while still inside its mother's womb. Full definition
The appearance of meconium passed in utero is a sign of fetal distress. (breedingbusiness.com)
Babies are born with a sticky tar - like substance called meconium in their intestines. (notmilk.com)
The study supporting this doesn't factor in the likelihood of post-date women trying castor oil, or the likelihood of post-date babies passing meconium in utero. (mamanatural.com)