Why do I need to know une tronche ?

Because you probably have some former classmates who fit this description

What does it mean?

Une tronche – roughly pronounced oon trohnsh –  normally refers to a big stump, log or block of wood, though not to be confused with the word “tranche”, which though it has a similar pronunciation, means a piece or slice of something. 

The French word for a block of wood has a second, casual meaning too – it can be used as a synonym for “face” or “head” – as it can in English too, such as in the phrase “knock your block off” to mean punch someone in the head.

You might be wondering now whether the word can be used synonymously with the English insult “blockhead” – referring to a dim person, but in fact it is the opposite. 

French people might call a genius or a highly intelligent person “une tronche,” similar to how English-speakers might call a very smart person a “brain.” 

Thus, a savant at mathematics might be referred to as une tronche en mathématiques.

Use it like this

C’est une tronche, il est le premier de la classe chaque année. – He is a genius, he is the top of the class every year.

Elle est une tronche dans l’apprentissage des langues, elle peut parler cinq langues différentes couramment.– She is a language-learning genius, she can speak five different languages fluently.