Origins: the infamous Crystalwank of FandomWank.

A popular internet catchphrase used to sarcastically denote obvious falseness or lying.

The full original quote, "MY HED IS PASTED ON YAY", comes from the above FandomWank thread wherein a LiveJournal user named Crystal attempted to convince her readers that she was real-life friends with a cast member of the Lord of the Rings films by making very obvious photo-manipulations of the actors in various poses in her room. These photos were quite clearly pictures of the actor's heads transposed onto different bodies.

The thread became monstrous in a very short time, eventually receiving over 13,000 comments. In the course of the thread, a commenter remarked that one of Crystal's photos wore a look of 'OmG my head iz PASTEDE ON YAY!!1111!!' The remark quickly spread across the Internet, where it was adopted and adapted. This FandomWank thread, especially the "pastede on" comment, is considered something of a Holy Grail of internet slang.

See also pastede, my hed is pastede on, yay.
Since the initial wank, anything may be regarded as "pastede on yay". A bad writer can be accused of having his/her spelling (or grammar, or ideas) "pastede on yay!" A fruity drink that does not taste particularly fruity might be said to have its "fruitiness pastede on yay!"
by LadySphinx November 26, 2004
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Definition: a mispelling of "pasted."

Background: See my hed is pastede on, yay!
by Nikkki November 23, 2004
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A phrase used to refer to the internet trend of people cropping the head of one person, perhaps a celebrity, and putting on the body of someone else, such as a naked person, via an image-editing program like Photoshop or Paintshop Pro. Often very obvious and ridiculously stupid. Originated via fandom wank. Spelled in the manner of illiterate 13 year olds.
13 Year Old Boy: Look, J.Lo naked! With Britney Spears!
Picture: My hed is pastede on, yay!
by Megan September 5, 2004
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A term used to describe an obviously Photoshopped or otherwise altered picture. It was coined by the user shinigami_co, and is still in rampant use. It appears to a permanent fixture in the Internet's vocabulary.
"Geez, this picture of a she-male Hilary Duff is so obviously of MY HED IZ PASTEDE ON YAY calibur."
by S. Chopstick January 5, 2005