From mixed Greek and Latin roots. Originally tecniculpus, combining the Greek tecn- (child) and Latin -culpus (fault). Lit. "faulty child". It had already morphed into tesiculpus by the time of the Venerable Bede.
There are alternative suggestions that the word stems from mistyping "rediculous" (sic) on a Qwerty keyboard. This appears unsubtantiated since Bede is not known to have used this keyboard layout.