Garp.
It comes from two places.
"The world according to Garp" by John Irvine. A really weird book, but quite funny too, in an obscure way. Well worth reading.
Second:
My old log in at sixth form college was gpr09907. One day we were really, really, really bored during a computing lesson (writing a program to read text in from file and convert the Roman Numerals contained within into decimal, before writing them to another text file.) We looked at each others logins and started taking the chronic p*** out of them, such as Adrian Brown, who had BRAsomenumberorother as his log in.
Someone figured out mine started GRP (it takes the first two letters of your surname and adds your first initial, before taking your admission number on the end of that), and said to me "Hmmm. so you're Gerp".
It was a fairly logical step from there, having just read the book.