Linguistics joke. There's a fake IPA symbol to go with it, that looks like a pig snout.
It's linguistically correct description of, well, the sound pigs make.
First-year linguistics students can be humiliated by asking them to include nasal-ingressive voiceless velar trill in their presentation of rare and difficult-to-pronounce sounds in the West African languages.
Recognized shorthand on the Usenet group comp.std.c for any unexpected behavior of a C compiler on encountering an undefined construct. During a discussion on that group in early 1992, a regular remarked "When the compiler encounters a given undefined construct it is legal for it to make demons fly out of your nose" (the implication is that the compiler may choose any arbitrarily bizarre way to interpret the code without violating the ANSI C standard). Someone else followed up with a reference to "nasal demons", which quickly became established. (see Jargon)
Someone who is motivated solely for himself. Lacks empathy for others. Is blind to others needs. Is incapable of hiding this true self for very long. He is tricky and has learned how to fake nice in social situations but his true self will show if he's needed for support in emotional situations or to act responsibly or compassionately.