Green’s Dictionary of Slang

(US campus) a general expression of nonchalance.

1981		Eble Campus Sl. Mar. 5: NBD – [...] ‘How was the frat party?’ ‘NBD.’.
1996		Eble Sl. and Sociability 37: Two kinds of shortening reduce words to letters. The dominant type in college slang is initialism, which names the individual letters: [...] NBD (no big deal).
2014		C. Eble (ed.) UNC-CH Campus Sl. Spring 2014.