Fly

Here are some idioms with fly (insect). The following idioms are easy to understand because they are based on the fact that flies are too small to notice (but still make most people feel unpleasant).

a fly in the ointment

The noun phrase “a fly in the ointment” means “some small circumstance which spoils the enjoyment”. This is totally understandable since dead flies in your ointment will definitely make you feel sick and you may stop using the whole ointment in the jar.

fly on the wall

This idiom means “an unperceived observer”. Your secret activity may be observed by the flies that you don’t notice the existence of.

drink with the flies

“Drink with the flies” means “to drink alone” or that drink. This seems to be used in Australian English, but is not quite commonly used.