The word 'stumper' is a straightforward agent noun formation from the verb 'stump', which entered English in the 16th century with the sense of reducing to a stump or rendering helpless. The verb's figurative sense of 'to baffle' or 'to puzzle' emerged in American English by the early 19th century, likely from the image of being brought to a standstill, as if confronted by an immovable tree stump. By adding the agent suffix '-er', the noun 'stumper' was formed to denote either the baffling problem itself or the person who poses it. The logic of the word's construction is transparent: that which 'stumps' (confounds) someone is a 'stumper'.

Structure: stump (to baffle) + er (agent suffix) - stump: From Middle English stumpen, stompen (to stumble, be nonplussed), of Germanic origin. In this word, it functions as the verb root meaning to baffle or perplex. - er: From Old English -ere, an agent-noun suffix of Germanic origin. In this word, it denotes a person or thing that performs the action of the verb.