How to Say "runt" in Spanish
The Spanish word for “runt” is “piojo” — B2 level.
English → SpanishB2
nounB2informal
a small or insignificant person

Examples
¡Vete de aquí, piojo!
Get out of here, you little squirt!
No era más que un piojo cuando lo conocí.
He was nothing but a little nobody when I met him.
Ese piojo se cree el dueño del mundo.
That little runt thinks he owns the world.
Figurative Gender
When used for a person, it's almost always 'un piojo' (masculine), even if the person is female, though 'pioja' is sometimes heard in very slang-heavy Latin American speech.
Tone Sensitivity
Mistake: “Using 'piojo' with a boss.”
Correction: This is very informal and can be insulting; only use it with friends or people you are looking down upon.
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