Somewhat great; fairly large or impressive, but not impressively great.
From 'great' plus the diminishing suffix '-ish' (from Old English '-isc'), which softens or moderates the meaning of adjectives to suggest an approximation or partial quality.
The '-ish' suffix is English's way of being wishy-washy—adding it is like shrugging linguistically. 'Greatish' is basically saying 'kind of great, maybe?' which is why it sounds a bit dismissive compared to 'great' itself.
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