Pethic is defined as a girl who has a small frame but a THIC Body, this meaning she has Big Boobs, Butt or Thighs, this word is a combination of the words "Petite" and "Thicc"
The Ryan Pethick Theory is an answer to the paradox of quantum superposition better known as "Schrödinger's cat". This theory states the second that the cat (or any other thing) becomes non-observable, it is dead. This theory is based on the not so logical logic that it is much easier to just assume that it's instantaneously dead. You don't have to worry about him, care about him, remember to send a text on his birthday. Basically everything is easier with the assumption that they are dead.