The story
Generally agreed to have originated in New Orleans, Louisiana, in the early 1920s. The explanation for exactly how the term arose is disputed, as discussed on Wikipedia at po' boy § Etymology.
Etymology from Wiktionary, CC BY-SA 4.0.
po' boy
Today it means: A sandwich made on a French bread roll cut lengthwise and usually containing a meaty filling (for example: roast beef, ham, or fried shrimp), and often dressed with ingredients like shredded lettuce, sliced tomatoes, sliced pickles, and mayonnaise. Full definition →