masc instr sg of thāmas (noun); masc abl sg of thāmas (noun)
Etymology. From √sthā 'stand, be firm' (Sanskrit sthira 'firm', English 'staid'). The form thāmas reflects the nominal derivative thāman-, which develops the concrete sense of 'standing firm, solidity' into the abstract 'steadfastness, resilience'. The adverbial thāmasā uses the feminine instrumental ending -ā to create an adverbial modifier: 'with/by means of firmness', hence 'obstinately'.
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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