Be one sandwich short of a picnic phrase
A pun on the phrase, taken from an advertisement published in The Age (Melbourne, Victoria, Australia) on 6 September 1992 for The Wharf food market, Southgate, Melbourne.
The phrase "short of a picnic a sandwich" indicates mentally deficient, or slightly crazy.
John is a sandwich short of a picnic. He gave up his lucrative job in order to pursue his passion for cooking.
That guy was obviously one sandwich short of a picnic. He always has many crazy ideas.
Used to describe someone who is naive, gullible, inexperienced, easily fooled, ignorant, unsophisticated, etc.
To say, or believe something or someone that seems completely crazy, delusional, or stupid
One who asks a fool question will receive a nonsensical answer
It is often used in all of tenses.
Penn & Teller is the earliest example I have found: no label on their bag of tricks, about the American magicians and entertainers Penn Fraser Jillette (born 1955) and Raymond Joseph Teller (born 1948), published in the Philadelphia Daily News (Philadelphia, Pennsylvania) on 15 May 1985.
A humorous way of saying that what one has said is just what they acknowledge and may not be entirely accurate
Mike: Have you returned from your vacation on the beach? Your skin seems to get tanned.
Jenny: My skin is always dark. That's my story and I'm sticking to it!