This is a slang meaning the same as having sex with a girl that has a period. A lot used among youths/teenagers in Kristiansand and the Southern parts of Norway.
Person A: "Æ kjørte brannbil i helga."
Person B: "Æsj! Det æ jo værre en å pule ho i ræva!"
Person A: "Æ blei kjent me brann sjefen."
Person B: "Herre Gud! Du æ faen ikke go!
Person A: "I drove firetruck this weekend."
Person B: "Ugh! That's worse than fucking her behind!"
Person A: "I got known to the fire chief."
Person B: "Oh my God! You are fucking disgusting!"
(Note: Kjøre brannbil means Drive a firetruck, in English.)