It is used as a way to express extreme surprise at a fact. Derived from the word "fathom" which simply means "to understand". Fathometh is used to mean "You understand?" or "Riiiight?" but in old English (or King James English).
Sarah: I can't believe Amywent back to her ex.
Jackie: Fathometh?
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).
Precedes or follows either a gratuitously disrespectful or honest remark, originated in New York City. Used in similar fashion to "no offense" or "to be honest," interchangeably.
Ya mans is wild buns, respectfully. Respectfully he's washed.
After school, The Modells on Jamaica Ave was active. Respectfully.