Khâgne

la khâgne = slang term used to designate the classes préparatoires (preparatory classes) to France's top schools ("grandes écoles")

This term refers in particular to humanities classes, but classes préparatoires more generally refers to the two years of intense study, right after high school, that lead to highly competitive exams for entrance into the top engineering, business and research schools, such as the Ecole Normale Supérieure (for research, aka Normale Sup', ENS or Ulm after the street where it's located in Paris) or Ecole Polytechnique (for engineering, also called "X"). Known for their intense competition between students ("no, you can't borrow my eraser, I wouldn't want you to correct your mistakes"), classes préparatoires also compete against each other. Among the most famous ones are Louis-le-Grand, Henri IV (aka H4), and Saint Louis, all located in Paris' famous Quartier Latin (latin quarter) student district, but many more exist throughout France.

By extension, un khâgneux / une khâgneuse = slang for a student in a preparatory class. Slang is actually an integral part of life in classes préparatoires, for example:

• maths sup' (short for mathématiques supérieures) = first year of math prep school
• maths spé (short for mathématiques spéciales) = second of math prep school
• 3/2 (trois-demi) = student in his second year of classes préparatoires
• 5/2 (cinq-demi) = student who's repeating his second year and taking the entry exams for the second time
• une colle = literally, glue. In university lingo, an oral test