Literally it means 'to put a harness on yourself', like упряжка.
Figuratively it's a slang word that means you are putting in a lot of effort or taking on an obligation, in this case:
Why are you 'white knighting' for this idiot girl? Why are you defending this idiot girl so much? or Why are you so worked up over this idiot girl?
Чо, чё, чиво, што, шо, штаа, чтаа are many bastardized variations of что you might see on the internet. They're all slang, including asking чего to show your bafflement, though чего is a polite word.