(US) Initialism of National Science Foundation.

(banking) non-sufficient funds

(Internet, initialism) Not safe (or suitable) for work; i.e., containing inappropriate or adult content (e.g. pornography or profanity) that might be offensive to employees and co-workers.

(Internet slang) Initialism of not suitable for minors.

Somewhat NSFW.

(Internet slang) Initialism of not safe for life.; i.e. containing graphic or repugnant content (e.g. fetish pornography, gore or lethal violence) that risks psychological trauma to those viewing it