A screw is a metal object similar to a nail, except it has a raised spiral line around it. Installing a ceiling fan requires use of screws. A screw helps fasten pieces together. The screws for the ceiling fan would need to be screwed upwards in order to install it. The installer is "screwing up" the fan, or installing the fan into the ceiling. "Screwing up" is a saying which means messing up. This joke is playing on the word "screwing up" and its different meanings.
How could I trust the ceiling fan installer when I knew he was always screwing up .