Nincompoops are people who are not very smart or do silly things.
Calling someone a nincompoop is not very nice.
Examples:
The nincompoops forgot their lunch at home.
Don't be such a nincompoop and put your shoes on the wrong feet.
The nincompoops made a mess in the kitchen.
The nincompoops tried to fly by jumping off the roof.
The nincompoops thought they could swim in the sidewalk puddle.
The nincompoops wore their clothes inside out to school.