The phrase 不亦乐乎 is formed by directly combining four characters: 不 (not), 亦 (also), 乐 (joy), and 乎 (classical question particle). It originates from a sentence in the Analects of Confucius, where it functions as a rhetorical question meaning "is it not a joy?" In modern usage, it has solidified into a fixed idiom, typically appended after 得 in structures like 忙得不亦乐乎 to convey an extreme degree, meaning "extremely" or "very much".