Resembling, suggestive of, or having qualities characteristic of a giant; extremely large or powerful in a somewhat strange or exaggerated way.
From 'giant' + '-ish' (a suffix meaning 'somewhat like' or 'having qualities of'). The '-ish' suffix comes from Old English and Germanic roots, productively used since Middle English to modify adjectives and nouns.
The '-ish' suffix is wonderfully flexible in English—it lets you express something that's 'almost but not quite' something else. Calling something 'giantish' suggests scale and power, but with a hint of oddity or approximation that 'giant-like' doesn't quite capture.
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