verb/fu.tʁ‿ɑ̃ l‿ɛʁ/
to screw up, mess up, fuck up, botch up something; to kill oneself
“Pourquoi foutre en l'air tout le mariage en me trompant avec ma sœur ?” — Why would you screw up the whole wedding by cheating on me with my sister?
Literally, “to fuck in the air”.