Right radical 卩 depicts a kneeling figure seen from the side. In 却 it pictures someone bending or stepping back, reinforcing the 'retreat' core meaning that later softened into the modern adversative sense (yet, however). Same component in 即, 节, 印, 危.
Left 去 - to depart, to leave. Supplies most of the sound (qù → què, a related rising reading) and a strong semantic flavour: 却 originally meant 'to step back, to retreat'. Same phonetic appears in 法 (law) and 怯 (timid).
