In a flint-like manner; with hardness, coldness, or unyielding firmness.
From 'flinty' (adjective) plus the adverb suffix '-ly' (from Old English '-lice', meaning 'in the manner of'). The '-ly' suffix is the primary adverb-forming tool in English.
The '-ly' suffix is so productive that English speakers instantly understand 'flintily,' 'splintily,' or 'printily' even if they've never encountered them—the suffix means 'in the manner of' so reliably.
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