Mundus inversus

Mundus inversus, Latin for "world upside-down," is a literary topos in which the natural order of things is overturned and social hierarchies are reversed. More generally, it is a symbolic inversion of any sort.

Although the words are ancient, the term mundus inversus has been common in English only since the 1960s.

In European Literature and the Latin Middle Ages, Ernst Robert Curtius first identified the topos, illustrating it with one of the Carmina Burana ("Florebat olim studium"), about which he comments (p. 94):

"The poem begins as a 'complaint on the times': youth will no longer study! Learning is in decay! But—so the thought proceeds—the whole world is topsy-turvy! The blind lead the blind and hurl them into the abyss; birds fly before they are fledged; the ass plays the lute; oxen dance; plow-boys turn soldiers. ... What was once outlawed is now praised. Everything is out of joint."

Curtius concludes with a formula for creating the mundus inversus (p. 96): "Out of stringing together impossibilia grows a topos: 'the world upsidedown.'"

In Renaissance-era French culture
In The World Upside Down in 16th-Century French Literature and Visual Culture, Vincent Robert-Nícoud introduces the mundus inversus by writing (p. 1):

"To call something ‘inverted’ or ‘topsy-turvy’ in the sixteenth century is, above all, to label  it  as  abnormal,  unnatural  and  going  against  the  natural  order  of  things. The topos of the world upside down brings to mind a world returned to its initial state of primeval chaos, in which everything is inside-out, topsy-turvy and out of bounds: the cart is set before the horse, only fools can be wise, kings no longer rule, the belly is placed above the head, people behave like animals, and  the  elements  are  at  war  with  each  other.  Widely  used  from  Antiquity  to  the  twenty-first  century,  the  topos  of  the  world  upside  down  is  extremely  versatile  and  can  be  applied  to  a  variety  of  situations.  Linked  to  adynata  or  impossibilia,  a  rhetorical  device  which  describes  a  natural  impossibility,  the  world upside down can be used to describe a dystopian or utopian world, and to  mock,  deplore,  criticise,  or  debase  a  person,  a  situation,  or  an  institution. The world  upside  down  can  be  found  in  poetry,  novels,  adages,  pamphlets,  paintings, and prints."

In anthropology
In the 1978 book The Reversible World: Symbolic Inversion in Art and Society (Symbol, Myth, and Ritual), folklorist Barbara Babcock defines mundus inversus as (p. 14): "any act of expressive behavior which inverts, contradicts, abrogates, or in some fashion presents an alternative to commonly held cultural codes, values, and norms — be they linguistic, literary or artistic, religious or social and political."