痛 (itai) is Japanese for "painful" while チャリ (chari -- short for "chariot") is Japanese slang for a bicycle. The name is inherited from similarly designed itasha -- originally "Italian Cars" but then the derisive meaning "painful cars" arose because they're so painfully embarrassing and painfully expensive. The otaku fanboys who decorate their cars and bikes this way then adopted the new painful meaning as a sign of ironic pride.
Click through the photos for attribution.
Here's another gallery of Itachari bicycles. Thank you to my pal Naoto for the Japanese language help.