Yoctoluminescent: really really dim or astoundingly dim. yocto is a prefix meaning 10 to the -24 power. luminescent means it gives off light. Yoctoluminescent therefore means gives off a very small light i.e. really really dim. By extension, it means astoundingly unintelligent.
An example would be "I guess we're lucky that yoctoluminescent tuber didn't drive a bus load of kids into the side of that train."