the act of typing flusteringly while talking to someone who makes you crazy
i was talking with my girl and i was so flustered i was totally fyping
Particularly useful in forums where photos of beautiful girls (or guys, if that's your inclination) are posted for the admiration of others.
It is a combination of "fapping" and "typing".
"She's gorgeous! If I make any speling mistakes, it's coz I'm fyping like a rabbit!"