What does "a few sandwiches short of a picnic" mean — and why is it funny?

informal, humorous

Meaning

A humorous way of saying that someone is a little foolish, eccentric, or not quite all there.

Where it comes from

British, popularized in the 1980s, part of a whole comic genre of 'short of a full...' phrases invented to insult someone affectionately.

Why it is funny

It measures a person's wits in picnic supplies. The phrase is so cheerfully absurd — you picture a picnic blanket with a suspicious gap where the sandwiches should be — that the insult arrives wrapped in a smile.

Used in a sentence

"She's convinced the moon landing was filmed in her garden — a few sandwiches short of a picnic."