NSA stands for no strings attached. A person looking for an "NSA" partner is looking for someone to hook-up and have sex with without the strings attached of a relationship. They definitely want to keep it casual and not serious.
NSA means no strings attached. Why buy the cow when you can get the milk for free type of deal. Just sex, no relationship, nothing complicated, no drama, just sex.
NSA would stand for no strings attached. This could mean someone is looking for a sexual relationship but doesnt want all the "strings" of a significant other.