In internet usage, particularly forum message boards, a user who swarms a post with useless commentary. Not considered a spam, as the user will usually post as part of a larger group of users, and the said user will usually think their comment is complimentary or in some way helpful, even though it is not.

The comment is:
1) Usually worthless. "OMG, that's great!" is not as complimentary as they poster believes it to be, and only serves to waste the time of the original poster.

2) Not well thought out. The comment is usually quick, and does not justify itself. No reason is given as to why the user feels the way he / she does about said topic, image, story, etc.

3) Part of a mob mentality. Piranhas don't work alone, unlike most spammers. Their posts are usually followed by a swarm of like-minded posts, fueling the useless flood of empty comments.

NOTE: These types of comments are usually most prevelant on a forum centered around skills development. (Art sites, writing boards, game modification boards, etc.) Where a person skilled in whatever the forum centers on is easily attacked by piranhas and his / her post is quickly devoured.
Original Poster: "Hey Guys, I just finished this 1200 DPI artwork, can you give some honest critique so I can improve myself?"

Piranha3083: "OMG! You are my favorite person ever! 111! Have my baby!"

Piranhaman111: "Doood! You have mad skillz! OMFG! 1337!

Prinininsenhna: "ROFLES. SO GOOOOD! I LOVE IT!"

Original Poster: "That doesn't help me at all, that was completely pointless!"

Piranha: "OMG!"

by Kyle Hinton March 12, 2006
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A member of Piranhas-Beach. A dragonboat club in Toronto
Hey look its a Piranha! That guy on Piranhas-Beach!
by Rob Chang August 12, 2006
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A group of 3 bitches or more, unsatisfied with their sexual life, that go out to a restaurant and just wait for the waiter to make the slightest, simplest mistake to devour him alive. The members of the group consist of the bitchy piranha, the bitchiest piranha, and the nice piranha who is willing to let you live.
PS. any attempt to appeal the bitchy piranhas will result in a catastrophic event, so for your safety and the sake of your tips, try to appeal to the nice piranha only.
Damn! i have a group of piranhas on table 21 that is taking most of my time.
by Pinche_patillas October 19, 2010
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eu quero procurar pirahnas oshe
by andy August 26, 2004
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A vagina...that is VERY tight. After insertion of penis, into previously mentioned snatch, the penis is overcome with the sensation of being in a very tight place. Thus likening the pussy to a piranha.
Man that girl that I banged last night definitely had a piranha... I don't know if my dick is still attached.

Made famous by Lil' Wayne in Keri Hilson's song "Turnin' Me On"
by BallsToTheWallz May 30, 2009
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Guys official action sports personal care products
Piranha X shampoo, body wash and hair fixatives.
www.piranha-x.com
by RosieMax January 12, 2012
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