English ⇄ Russian

sleeper build stresses
mach.mech.	скрытая сборка (The term is often used in the context of customizing or modifying a vehicle (such as a car) to make it appear unassuming or stock on the outside while enhancing its performance. Artie_Osipov)
sport.	Спортсмен-темная лошадка (Artie_Osipov); скрытый спортсмен (The term "sleeper build" in the context of people typically refers to someone who may not appear impressive, talented, or skilled at first glance but has hidden or latent potential, abilities, or qualities that can be developed or awakened over time.) Artie_Osipov)