The story
Partial calque of Cantonese 噏乜春 (ngap¹ mat¹ ceon¹, “what the hell are you babbling about”, literally “what testicles are you babbling about”), where 春 (ceon¹) is a homograph meaning both “testicles” and “spring (season)”. By surface analysis, Cantonese up + what + spring.
Etymology from Wiktionary, CC BY-SA 4.0.