No matar ni una mosca

not to hurt a fly; **not to hurt a fly**; **to be as gentle as a lamb**; **to be completely harmless**; **to have a heart of gold** (in some contexts)

No matar ni una mosca means to be completely harmless, extremely gentle or incapable of hurting anyone.

It often refers to someone's peaceful or timid nature.

• is very kind and gentle
• seems incapable of violence
• is timid or soft-hearted
• has been unfairly suspected of doing something bad
• gives the impression of being completely harmless

The expression is based on deliberate exaggeration.

A fly is one of the smallest and least threatening creatures imaginable.

If someone would not even kill a fly, they are presented as extraordinarily peaceful and harmless.

The same metaphor developed independently in several European languages.

The expression often describes people who appear shy, quiet or good-natured.

However, Spanish speakers sometimes use it ironically when someone who seemed completely harmless unexpectedly behaves badly.

For example:

Pues mira, parecía que no mataba ni una mosca...

This introduces a contrast between appearance and reality.

Spanish has several idioms describing personality through animals:

• No matar ni una mosca → not to hurt a fly.
• Ser un lince → to be very clever.
• Ser un gallina → to be a coward.
• Estar como una cabra → to be crazy.

Among them, no matar ni una mosca is one of the warmest descriptions of a person's peaceful and gentle character.

Unlike many Spanish idioms, English has an almost identical expression:

• He wouldn't hurt a fly.

This is by far the most natural translation.

Other possibilities include:

• She's completely harmless.
• He's as gentle as a lamb.

The emphasis is on the person's peaceful nature.