Apo
Bengali word for sister. Except you don't use it unless you love your sister too much. Just like Jaime Lannister loves his sister Cersei Lannister in Game of Thrones. You could use this in a sentence like "Thanks apo apnio" which conveys the utmost form of gratitude towards the apo you are addressing.
Example:
Girl: Wow! You make killer memes!
Boy: Thanks apo apnio.
Girl: Don't come near me or my city ever again.
Girl: Wow! You make killer memes!
Boy: Thanks apo apnio.
Girl: Don't come near me or my city ever again.