Hogwash did not begin as an insult. It originally referred to actual feed for pigs, especially a watery mix of kitchen scraps, leftovers, or swill given to hogs.
Over time, the word picked up other meanings. By the late Middle Ages, hogwash could also refer to poor quality liquor. That sense probably grew from the idea of something weak, sloppy, or low grade.
Eventually, the word took on the figurative meaning most people know today: nonsense or worthless talk.
- Literal use: “The farmer poured hogwash into the trough.”
- Older extended use: “That tavern serves hogwash.”
- Modern use: “That rumor is hogwash.”
This kind of shift is common in English. A word can start with a concrete meaning, then move into a more judgmental or metaphorical one. In this case, something seen as low quality became a handy way to dismiss an idea as foolish or untrue.