Usage notes: The literal meanings of the French expression chercher des poux dans la tête à quelqu’un and the English expression “to nit-pick” are equivalent, but their idiomatic meanings are somewhat different.
In French, the figurative meaning of chercher des poux is to look for a fight, search for something – usually very minor – to argue about. There is a nuance of aggressiveness that is missing in English.*
Qu’est-ce que tu as ? Tu me cherches des poux dans la tête depuis deux heures !
What’s the matter with you? You’ve been trying to pick a fight for two hours!
Attention avec ce mec-là. Il vaut mieux ne pas lui chercher des poux.
Be careful with that guy. It's better not to quibble with him.
* In English, "to nit-pick" means to split hairs, get caught up in tiny details but without actually fighting about them. This meaning is better translated by chercher la petite bête or couper les cheveux en quatre.