Resembling or characteristic of a chump; foolish, stupid, or blockheaded.
From 'chump' (a foolish person) plus the suffix '-ish,' which means 'somewhat like' or 'having qualities of.' The '-ish' suffix is productive in English for creating adjectives from nouns.
Words ending in '-ish' are English's way of being a little mean—'chumpish' is just insulting enough to be funny while maintaining plausible deniability.
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