what gets thrown out: the worthless leftovers swept away once the useful part has been taken
This word names the discarded remainder: the siftings left in a sieve, the chaff after winnowing, the dregs in a cup, the scraps cleared from a table, the filth washed through a street, and (especially in ancient doctors' writing) the body's waste. The single thread running through all of it is the same: this is what is left over after the good part has been pulled out, and it is now fit only to be carted off.
Paul uses it once, at the climax of Philippians 3. He has just listed his most prized credentials, the things that made him a model of Jewish piety, and then he stacks up an accounting image that keeps getting starker: he counts them as loss, says he has lost them all, and finally calls them this. The point is the reversal of value. Set beside knowing Christ, even his finest religious assets are the residue that gets swept off the threshing floor.
Much ink has been spilled over how crude the word is. It clearly carries disgust, because the things it points to are often disgusting. But it was also the polite, ordinary word that physicians and historians and even wisdom writers used, and a handful of real men were named with it, so it is forceful and vivid rather than a true swear word. English Bibles land all over the map for exactly this reason: dung, refuse, rubbish, garbage, filth.