Guitarded is usually (and at times, considerablly offendsive) slang for a person with some form of retardation playing a guitar, however it does vary. Guitarded could also define a mentally normal person that isn't good at the guitar, or a person that lacks knowledge on music.
It could also refer to guitar center, a popular chain in the US for instruments. Keep in mind, when referred to as "Guitarded center", the person using the word is usually not fond of guitar center (Personally I don't mind the store, but many people dislike it).
1: guitarded as in person with retardation playing guitar:" Hey Mikey, Did you know that Billy with the downs
is learning the guitar?"
" Ah...so he's guitarded now, right Jeffrey?"
2: Guitarded as in a person that lacks musical knowledge.
"Dude. You don't even know what a Fsharp/D chord is!? And you've been playing for three years? No offence, Mikey, but you're guitarded."
3: Guitarded as in "person that is poor at playing the guitar": " Says Mr. Jeffrey who's so guitarded that he can't even play the G chord!"
4: Guitarded as in a reference to the music store chain Guitar Center: "Mike, I'm sorry." "I'm sorry too, Jeff. How about we go kill some time at Guitarded center?"