Said when someone’s a bit daft or spaced out, like they’re missing a key piece of the kit. It paints a silly picture of turning up for a picnic with the blanket, crisps and drinks, then realising you forgot the actual sandwich. Usually cheeky banter rather than proper cruelty, but it still means they’re not firing on all cylinders.
"Dave tried to iron his shirt while still wearing it, then blamed the kettle for the burn. He’s one sandwich short of a picnic, bless him."