APAD: Believe you me

"Believe you me" inverts the colloquial syntax of the imperative statement "Believe me" by placing the implied vocative "you" between verb and object rather than before them ("You, believe me"). It is an emphatic way of saying "believe me" or "mark my words." It is used to stress that what you are saying is definitely true and that the listener should take it seriously. 

It serves as a more intense version of "believe me" or "trust me," often used when the speaker is sharing something extreme or unexpected.

While it looks "backwards," it is a rare surviving example of an archaic English imperative where the subject ("you") follows the verb. Similar structures are found in older literature like the King James Bible ("Hear ye me") and Shakespeare ("Mark you me").

Some linguists believe its widespread use was influenced by Irish English (Hiberno-English), mirroring Gaelic sentence structures.
It is primarily colloquial and used in speech rather than formal writing. 

"It’s going to be a long day, believe you me!"