It seems that in the 21st century the definition of "slouching" is offered not as a word, but in the "social" power of language, and in this era, the "slouching" meaning is centered on an individual's "posture."
Are you wondering whether the words we use are changed, or adapted by some unseen power that wants to shape the way the world is viewed? I found the Stanford University article, which I have listed, that actually discusses why language is undergoing a structural change. And the title states the "why" succinctly, "the-power-of-language."
Is it possible that without our knowledge the power of language is unleashed by the education establishment?
All of the above should be in mind if you live in the USA.
The nations worldwide guard their cultural with language, there are no changes, only in the USA do we let our "betters" decide how we change our words to benefit those who seek to control "the power of language."