where you had a throbber, and then has died down, but is still not gone flacid, and is still erect, but has just lost most of the inflation, giving the impression of a pencil dick, but you've just not got a full-on boner.
i was having a wank the other day, and then i heard a noise, but then when i went back, i was at half mast
Has several meanings, but used in a situation where someone has done/said something retarded, and so "derp" is a socially acceptable to say post-retardedness.
This meaning is most commonly used (originally, atleast) in Coventry, England.
Pronounced "durrp." See "yarp" from the film "Hot Fuzz" The way Michael says "yarp" is similar in the way you say "derp."
Person 1: Omg, i thought i saw a cat, but it was a dog, but yeah. I failed alll my gcses.
Person 2: Derp. (retard face)