Somewhat like a geyser or tending to behave like a geyser; characterized by sudden, forceful outbursts resembling geyser eruptions.
From geyser plus the suffix -ish, which makes adjectives from nouns and implies 'somewhat like' or 'having qualities of.' This follows the productive English pattern of using -ish for descriptive purposes.
Calling something 'geyserish' rather than 'like a geyser' gives it a playful, almost personality-based quality—you might describe a temperamental person as geyserish if they erupt unpredictably!
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