The English word "disorder" is seldom-to-never used in a way that means "the measure of a system's thermal energy per unit temperature that is unavailable for doing useful work." (Encyclopedia Britannica def. of entropy).
Needless to state that this definition would be completely useless for someone that didn`t already know what entropy is to begin with.
Answer me this question: How does that definition accounts for the increase in entropy that happens when you mix to different gases? What about the increase in entropy that happens whn you shuffle a deck of cards?
Entropy is not a measure of a system's thermal energy per unit temperature that is unavailable for doing useful work. It doesn`t even have the right units to be a measure of energy (That might be more apropriate as a definition for free energy, but even there, only in a lose sense).
Entropy is a measure of the number of microstates (a complete description of the state of the system) of a system consistent with a given macrostate (a thermodynamic description of the system). Since there are many more states consistent with disorder then there are states consistent with order, it seems not entirely inapropriate to identify entropy with disorder.