The word 差劲 is formed by combining two characters: 差, which carries the core meaning of "poor", "inferior", or "falling short", and 劲, which originally means "strength", "vigor", or "force". The formation works conceptually by using the image of strength or effort (劲) as a benchmark; when that strength or the quality of that effort is deemed poor or lacking (差), the resulting compound evokes the idea of something that is disappointingly weak, ineffective, or of low quality. It’s a vivid, almost metaphorical construction where "poor strength" translates to being unsatisfactory or no good.