Something or someone that is remarkable, excellent, or impressively large; in older slang, someone or something that 'corks' the argument by settling it conclusively.
From 'cork' + agent suffix '-er', emerging in American English by the 1880s-1890s. Originally referred to something that was the final convincing element, like a cork that seals a bottle completely.
The evolution of 'corker' from literal cork-makers to 'that's a real corker!' (excellent!) shows how slang hijacks random words when they perfectly capture an idea—by the 1920s-30s, jazz musicians were calling amazing things 'corkers'.
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