Flop sweat refers to the cold precipitation that starts creeping out of one’s pores following a great embarrassment.
Flop itself is a slang term for failure which brings with itself great embarrassment, like the Cone of Shame.
Although the exact origin of the phrase “Flop sweat” is not known, one of the earliest documented cases of it appearing in print can be found in a 1941 issue of the Chicago Stagebill.
Numerous other cases of the expression have appeared in the 1940’s, especially in magazines and newspapers.
During the second half of the 20th century, the phrase gained a larger prominence, appearing in print, as well as television and radio.
“Flop sweat” was first defined on Urban Dictionary on March 7th, 2015, with several other entries following in the subsequent years.