Means solid. Also spelt solerrd, solerrrd, solerrrrd and so on, depending on the solidity of the subject. Often used in Milton Keynes, possibly because the solidity of people in Milton Keynes required a new word to describe them.
Mike: Willy is solerd!
Olay: Yeah, he drinks beersometimes!
Mike: Solerrrd!
A solcredulist is a word to describe someone who believes everything that is written in the Sun newspaper. The word comes from the book 'Stone Cold' by Robert Swindells.
Girl: Omg, he believed that sh*t written in the Sun?
Boy: Yes, of course, he's such a solcredulist!