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.