Senses
挥汗成雨/ 揮汗成雨 to sweat profusely
to drip with sweat, meaning literally that wiped-away sweat falls like rain
挥汗成雨/ 揮汗成雨 crowded
describes a place with so many people that their collective sweat would form rain, meaning very crowded; often used alongside 张袂成阴