In thermodynamic literature, mixed-up-ness is a oft-quoted term that American mathematical physicist Willard Gibbs supposedly referred to entropy as. The term, however, is only found as a fragment header sentence in the unpublished manuscripts of Gibbs, that he was supposedly to do or expand on in a future reprint of On the Equilibrium of Heterogeneous Substances, as shown adjacent.

Example
In the 1920 A System of Physical Chemistry by William Lewis and James Rice, state that “the molecules of a gas are in a continuous disordered movement, a gas being in fact a molecular chaos”. [1] They refer to Gibbs' description of entropy as mixed-up-ness and state that “this definition of entropy will be understood to a certain extent if we think of a substance as a molecular chaos … owing to collisions between the molecules their motion tends to become more and more disordered until a final stage of disorder is reached.”

The section heading “entropy as mixed-up-ness” by Gibbs, however, was a planned, but never finished, chapter heading found in the unpublished fragments of Gibbs papers. [2] In any event, they incorrectly state that “Gibbs considered that the degree of the disorder was identical with entropy” and that “when the disorder or chaos is greatest the entropy of a substance is likewise a maximum”. Gibbs, however, never once used the words “disorder” or “chaos”. [3]

References
1. Lewis, William and Rice, James. (1920). A System of Physical Chemistry, (pg. 48). Longmans and Green.
2. Mixed-up-ness (in the collected Scientific Papers of J. Willard Gibbs (Longmans), the reader will find on page 418 of the first volume a number of unpublished fragments, one subject bearing the title: Entropy as mixed-up-ness, a planned, but never finished, chapter).
3. Google book search of The Collected Works (Thermodynamics) and Elementary Principles in Statistical Mechanics.