The word "买单" is formed by combining the character "买" (mǎi), meaning "to buy", with "单" (dān), meaning "bill" or "list", creating a literal sense of "buy the bill". It originates from the Cantonese phrase "埋单", where "埋" means to settle, and in Mandarin, "买" was adopted to convey a similar idea of settling payment, resulting in a compound that specifically refers to paying a bill, often in a restaurant context.