a fly on the wall Definition

fly¹
noun flies

1. A two-winged insect, especially the common housefly.
2. Any of various other flying insects.

Example: mayfly
Example: dragonfly
Example: butterfly
3. angling.
A fish hook tied with colourful feathers to look like a fly, used in fly-fishing.

Idiom: drop like flies

colloq
To fall ill or die in large numbers.

Idiom: a fly in the ointment

A drawback or disadvantage to an otherwise satisfactory state of affairs.

Idiom: a fly on the wall

The invisible observer, usually at a meeting or in a social situation, that one would like to be on certain occasions to find out what is happening without having to take part.

Idiom: no flies on someone

colloq
Referring to someone who is cunning and not easily fooled
Example: There are no flies on Harry.

Idiom: they etc wouldn't harm a fly

They etc have a gentle nature.

Etymology: Anglo-Saxon fleoge.