adverb
impetuously; eagerly; ardently
Poetic adverb expressing intense, headlong eagerness or impatience. Rare in everyday speech; found in classical or literary contexts.
彼はやたけに戦場へ駆け出した。
He dashed impetuously onto the battlefield.
やたけに恋い焦がれる思いを歌に詠んだ。
She composed a poem about her ardently yearning heart.
The exact derivation is uncertain. The kanji 弥猛 suggest 'increasingly fierce,' but the word is likely a fixed poetic expression whose components are not transparently productive in modern Japanese.