Most comfortable; superlative form of comfy, meaning the most pleasant and easy of all options.
From 'comfy' + -est superlative suffix. 'Comfy' arose in the 1800s as informal British English shortening of 'comfortable,' from Latin 'confortare' meaning to strengthen.
The word 'comfiest' shows how English is perfectly happy to have both formal ('most comfortable') and informal ('comfiest') versions of the same meaning—one for writing essays, one for talking to friends about your favorite sweater.
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