The new, avant-garde way of saying the word "old." Originates from the fact that clams have shells. Also spelling words with 'z's just makes them that much cooler. For more emphasis, try using it in capitals, i.e.: CLAMSHELLZ. Remember, never EVER separate the two words, as some cultures find the separated words extremely offensive.
OK Go's dancing, sadly unlike Alex Kapranos, will never grow CLAMSHELLZ.