(as) useless as an ashtray on a motorbike

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(as) useless as an ashtray on a motorbike

Utterly ineffectual or unnecessary. Primarily heard in Australia. Hey, thanks for the broken wrench—it's as useless as an ashtray on a motorbike. I know next to nothing about computers, so I'd be useless as an ashtray on a motorbike trying to figure out what's wrong with your laptop.
See also: on, useless