In literal terms, it is the act of defecating against the current of a body of water because the circumstances of the situation offer the individual no other option but to poop right then and there in the river. This messy situation, having to poop while submerged in water, is exacerbated by the poor choice of the individual to defecate with his/her backside facing upstream. The current then pushes the feces back in the direction of the defecator and thus makes this already messy situation much messier. In an everyday scenario, this expression means to attempt to improve a desperate situation but in doing so, make the situation worse for oneself.
Dude 1: How’d it go with Kelly last night? You finally get to bone her before you left for school?
Dude 2: No, I made it worse.
Dude 1: How?
Dude 2: Got myself into the friend zone. I heard she was more into sensitive guys so I tried really hard to be sensitive around her. She ended up telling me I'm a really good person and we should just be friends. I was shitting upstream, man. Now there’s no chance we’re gonna bone, even when we come home for winter vacation.
The experience of inexplicable familiarity felt when seeing a bro in a social setting (e.g. a party). This familiarity is often inexplicable due to excessive consumption of alcohol by both parties. But following a conversation between the two bros and some hard thinking, it is usually surmised that they met in class, saw each other on campus, or ran into each other in some other random setting. This often leads to good-natured conversation about their limited commonality over drinks.
Bro #1: "Bro, I've got total deja bro right now? Where have I seen you before?"
Bro #2: "Yeah same, dawg. Did we have a finance class together or something?"
Bro #1: "Yeah! Now I remember, I was mad hungover when you did some dumb ass presentation or something!"