Abreviation of Frustrated IRC User. Not only used to describe flaming, annoyed or sexually frustrated (all the same) people on IRC but also used for these kind of people in the real world.
A fircu can be detected easily by regular people; a fircu usually has an obsession for something, has problems comunicating and/or usually overreacts extremely when someone ask him something normal.
The word originated after heavy discussions on IRC
Example 1: Jim at a store:
Jim> Can I get a screwdriver?
Retailer> What kind of screwdriver do you want, i got 40 different screwdrivers. I have the Philips 1, 2 and 3, flat, torx. Don't bother me with your ignorance about screwdrivers and say exactly what you want. I do NOT want to waste my time explaining to you.
Example 2: You on IRC:
Mike_1> m3h pwnd at GTA-SA
Mike_1> You are all t3h suxor
You> Stfu fircu
The retailer from example 1 or the irc user called 'Mike_1' in the second example are perfect illustrations of a Fircu.