Rest in peace is a common epitaph found on Western gravestones. In colloquial speech and writing, it's used to note the literal or metaphorical death of someone or something.
How you doing is an informal greeting like How's it going. It is most commonly known as the signature pickup line of character Joey Tribbiani from the sitcom Friends.
BMS is an internet acronym for “broke my scale," used in support of a user, often a woman, posting a picture of herself and asking for a numerical rating from her online followers.