In the 2001 book Producing Animation, Catherine Winder and Zahra Dowlatabadi explain:
The origin for the term "sweat box" is said to date back to when Walt Disney would view the scenes completed through rough animation with his animators and critique their work. Some attribute the word "sweat" to the fact that screenings took place in a small theater and it got hot, while others believe that the animators would actually sweat in response to how Disney might react to their work. Either way, the same wording is used today when a scene is ready to be approved by the director in stages of rough animation, clean up and effects animation, and final color.3
Winder, Catherine; Dowlatabadi, Zahra (2001). Producing Animation. Amsterdam: Elsevier. p. 224. ISBN 9781136132612. Retrieved 9 September 2020. 9781136132612 ↩