The Rice University Neologisms Database

No big deal; unimportant, unnecessary; something not worth worrying over. 'NBD' is an acronym formed by the first letters of 'no big deal'. It was probably first formed for ease in texting or typing to represent a frequently used phrase, just as 'lol' and 'BFD' formed, and then people began to take it from written context and use it in actual conversations simply as a form of verbal shorthand or because they knew the general meaning but not what the actual letters stood for. It is very informal and typically not used as part of a sentence and stands on its own and is most often used among people frequently on the internet or texting.