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.