The expression 'one sandwich short of a picnic' can be used in a humorous way to refer to someone who is crazy or stupid.
John is one sandwich short of a picnic. He gave up a job in a big bank to live in a caravan.
When we arrived at our hotel by the beach in Miami, all Mary had in her suitcase was her fur coat. She is one sandwich short of a picnic.
Don't confuse it with
The expression 'to be the meat in the sandwich' means to be in a difficult position, as the person between two others that are arguing.
My brother and my sister are fighting over who gets our late mother's cat. I'm the meat in the sandwich, as usual, stuck in the middle.
Halloween marks the end of the Celtic year and was believed to be the day when the spirits of those who died in the previous year would come back and possess a body of the living to allow them passage into the afterlife. To frighten them and avoid becoming possessed, the living would dress up in scary costumes.