鴻蒙 hóngméng is an off-rhyme. 鴻 is a kind of goose and also means "large." Given Zhuangzi's interest in large birds, both meanings are probably at play. in 14.01, 蒙 means 'obscured' or 'ignorant.' Later on, it occurs in a few peoples names. Here, notably, it is part of a rhyming reduplicative, hence present for sound as much as for meaning. Also, note that it is almost homophonous with 芒 máng, which has a variety of meanings suggesting confusion.