A gamertag is a person's online persona over xbox live.

You need to pick it wisely as Microsoft will try to get as much money from you as possible because it costs 10 dollars to change it.

Also there are unwritten rules when it comes to creating your gamertag.
Rule #1: It may contain at least 1 marijuana reference.
Rule #2: It may or may not (your choice) reference alcohol or boobs.
Rule #3: It must have 3 x's somewhere in the gamertag (perferably at the beginning and the end)
Rule #4: If you are 12 years old and are planning on playing halo 3 all day after school until your bedtime you must put MLG in the beginning of your gamertag.
Rule #5: You must, MUST make it look you're so retarded that you spent your whole time as a baby in a washing machine.
Here are some examples of gamertag you might want to use.
xXx LeGiiT SniiPeZ
Sw4gga like dis
iTZ iLoGiCzzz v2
KSI Sn1p3D0vv/V

The list is endless
by BartS4177 January 25, 2010
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My gamertag is Funperson123, send me a freind request on xbox live.
by JasonT January 1, 2006
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An online persona over the popular Xbox LIVE feature on the Xbox 360. You have to have a maximum of 15 characters consisting of numbers and letters, but it must start with a letter.
x iCaMPeR
x SaTaNiiCc
II TaLeNTz v3
x NeRv3 x
x1337 N05c0p35x
iTz 2 Pr0
o iPRoShoT xX

"I got a new gamertag today."
"My gamertag is so pro x MLG (joke)"
"Whoa, that gamertag is leet"
by Spanky McShif February 22, 2009
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A Gamertag is commonly known as a Username online on consle or pc but it is more commonly used on consle.
friend 1: Wow that guy has a very cool gamertag.

friend 2: Yeah I think so too!
by intuectuall January 10, 2020
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An Xbox or PS3 made "Online Account" with a word from the dictionary pretty much, Basically any word made into a name is an "OG Gamertag" Example: Dog, Cat, House, Truancy etc.. HAVING AN X BEFORE A NAME OR 4 LETTER THAT MAKE NO SENSE IS NOT OG.
Jarrod: anyone want to buy my OG Gamertag?? its xJarrod
Dave: that's not OG you fucking rand, Kys
Josh: My OG is better, proons
Dave: that's still not og you idiot
Jarrod: Yea? lets see yours Mr. OG Dave
Dave: mines Failure so smd
by Acrnn December 1, 2016
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It is a funny joke of altering the layout of your gamertag on Modern Warfare 2 to look different. You and make it say something funny like "I R Pro" in blue or say "MLG is Gay" in pink and black. You can also include other charaters like X, A, B, Y, RB, LB, R, L, RS, LS, Dpad buttons in there.
"Dude your gamertag is so annoying with those LB and RB buttons. When you talk they flash on and off all over my screen."

"I really hate faggots who think they are so cool by using modded gamertags."

"Hey kid how'd you do that with your gamertag?"
"Tell me!"
"No shut up!"

20 random messages later...
"Please tell me! I'll give you Reecon."
by ShiningValkyrie April 23, 2010
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The act of actually taking a shit on your own Xbox console that is connected to Xbox Live.

More commonly, it is the act of making fun of a gamertag online.
by GARY FATTY October 29, 2010
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