Stupid

bright spark

Meaning: an intelligent and lively person (humorously)

Example: Emma is the bright spark of the school – she scored the highest marks in the class. Read on

put your foot in it

Meaning: say something (by mistake) that upsets, humiliates, or embarrasses someone

Example: Carla put her foot right in it when she congratulated her neighbour on being pregnant. It turns out she's not expecting but had just put on weight. Read on

not playing with a full deck

Meaning: not mentally sound

Example: With the number of silly remarks he comes up with, I wonder if he's not playing with a full deck. Read on

lights are on but nobody is home

Meaning: used to describe a stupid person

Example: I tried all to understand him, but the lights are on but nobody is home. Read on