Not "all there in the head"; possibly lacking intelligence or common sense. The wall-eyed background character in "My Little Pony: Friendship is Magic", Derpy Hooves, is named after this.
Hey, look at that guy sticking his aluminum foil-wrapped dick into an electrical outlet. That's derpy if I've ever seen it.

Derpy is best pony.
by datSato December 31, 2011
A highly contagious disease originating from John Lane.
This disease is spread through acts of sheer stupidity, and there is no known cure.
Dammit, John touched me funny style and now I have da derpies
by RamstainAU September 26, 2010
A pony that is from My Little Pony
I watched My Little Pony and my favorite pony is Derpy
by Cat Lover50 December 09, 2015
Derpies is an increasingly prevalent socially-transmitted disease that impairs mental function in afflicted individuals. Derpies has become as common as one-in-two individuals on average in the world's population. The level of concentration of affected individuals increases the risk of exposure and transmission exponentially. Despite this fact, rural areas appear equally affected as urban areas. Cooperative vigilance and increased awareness are strongly advised as the only effective deterrents to the spread of this illness.

Symptoms of Derpies include:

Decline in common sense
Impairment of motor functions
Reduction in observational skills
Appreciation of reality television
Pattern of poor decisionmaking
Increased laugh response
Reckless driving and parking
Misuse of commonly used words
Distortion of spacial reasoning
Excessive quasi-aggressive
physical contact

If four or more of the listed symptoms apply to you, a friend or a loved one, Derpies may have already taken root in your social environment. Unlike its permanent, sexually-transmitted
cousin; Herpes, the non-permanent Derpies is potentially deadly. It is important for affected individuals to seek academic assistance immediately to mitigate Derpies-associated
risks to your social well-being.
Bob: "John has been acting ridiculously. He likes stupid things and laughs all the time."
Alice: "He has been hanging out with Frat boys."
Bob: "He has been watching the Jersey Shore reruns."
Alice: "We need to have an talk with him. I think that he has Derpies. And, I don't want to get it to."
by PacketLost August 06, 2015
Uncontrollably horny female with a hunger for dick, ready to take it in any hole at anytime.
Did you see that girl brought a 12 pack of condoms with her to the party? "Yea she's real derpy"
by Terez Slowens November 24, 2015
Being just the slightest bit off. Having a clumsy way about it.
" That squirrel is running sideways, he's pretty derpy."

" Kristen Stewart's facial expressions are so derpy, it's painful to watch."
by UnicornPornHorn March 30, 2015
A character, or enemy in a game who is terrible and can beat noone. This does not apply to players. A player of this description is known as a noob.
Joe: man I love Minecraft squids

Tom: why?

Joe: cause they're so derpy
by friendofthewiz July 07, 2015
(noun) 1. the unfortunate condition of permanent ugliness whose contorted nature is often misconstrued with the common phenomena of "herp-derping"
2. a title for someone who is often seen "herp-derping," whether in photographs or in real life
3. a good way to troll, or fool, others into admitting to embarrassing and most likely untrue diseases
1. Jake: "Dude, what's up with Todd's face?"
John: "I don't know man, he looks like he's herp-derping."
Todd: "Herrrrrrrrp, I have derpies."

2. Sam: "You ever notice how Todd is always derping in pictures?"
Chris: "Yeah, it's weird, I wonder if he has derpies."
Todd: "Herrrrrrrp, I have derpies."
Sam: "You don't say...."

3. Troll: "Herp, I have derpies."
Victim: "Oh yeah? Well, derp, I have herpies."
Crowd: "Booh!" "Get off the stage!"
Victim: "FFFFFFFFFFFUUUUUUUUUUUU...!"
by githeroter December 02, 2012

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