What it means
A light, teasing way to say someone's a bit daft, not fully switched on, or missing some common sense. The image is simple: if a deck's missing cards, it isn't complete. Usually said with a grin, not full-on nasty.
Usage examples
"Lovely fella, but a few cards short of a deck, he watered the plastic plants for a month."
"Bless him, he is a few cards short of a deck, but his heart is in the right place."
"Anyone who paints a fence in the rain is a few cards short of a deck."
"He's a few cards short of a deck if he thought the satnav meant drive into the lake."
"My cousin's a few cards short of a deck, bless him, tried to charge his phone in the microwave."
Where it comes from
This belongs to a big family of English jokey put-downs built on something being incomplete, with the sense recorded in the late 20th century. The card deck image does the heavy lifting: if a few cards are missing, the pack isn't working properly. It's closely related to not playing with a full deck.
