fetus friends
2 or more friends who are unconditional best friends from birth, who know each other inside and out. they're usually kind of annoying
those two are fetus friends
Fetus friend
A friend that you’ve known since the infancy or a childhood friend that you’ve known your entire life, essentially since the time you we’re both fetuses, because your mom and their mom and/or parents have been friends since before your birth and you became friends with your parent’s friend’s child by default.
I was hanging with my fetus friend last week, the one I knew since I was basically a fetus.