Question

Have a Screw Loose to behave or look in a strange or foolish manner; to be odd and not ordinary

Accepted Answer

That substitute gym teacher must have had a screw loose. During the machine age beginning around 1860, many contraptions were held together by nuts, bolts, and screws. If a screw came loose, the machine would not operate as it was supposed to. It might start to do all sorts of bizarre things. People aren't held together with screws, of course, but if they start acting weirdly, this expression might fit them perfectly.

Idiom	Have a Screw Loose
Example	That substitute gym teacher must have had a screw loose.
Meaning	to behave or look in a strange or foolish manner; to be odd and not ordinary
Origin	During the machine age beginning around 1860, many contraptions were held together by nuts, bolts, and screws. If a screw came loose, the machine would not operate as it was supposed to. It might start to do all sorts of bizarre things. People aren't held together with screws, of course, but if they start acting weirdly, this expression might fit them perfectly.

Screw Loose Machine Held Start Substitute Gym Teacher