taru-adjective, adverb taking the 'to' particle
absolute; complete; utter; pure; sheer
Used as a たる adjective or adverb with と to emphasize that something is entirely, purely, or unmistakably the case. Often found in formal or written language.
それは純然たる事実だ。
That is an absolute fact.
彼の行動は純然たる犯罪
His actions are sheer criminal conduct.
この問題は純然たる技術的な問題だ。
This problem is a purely technical one.
Compound of 純 (pure) and 然 (a suffix forming adjectives/adverbs indicating state or manner). The exact historical derivation is uncertain, but the combination conveys 'in a pure state' or 'unmixed'.