adverb taking the 'to' particle, taru-adjective
fiercely; ferociously; furiously; resolutely
Describes a sudden, intense, and forceful action or state. Often used with verbs of movement, attack, or determination. The 'to' particle is typically required when used adverbially.
犬が猛然と吠えかかってきた。
The dog came at me, barking fiercely.
彼は猛然と反対した。
He opposed it resolutely.
Compound of 猛 (もう, fierce) and 然 (ぜん, a suffix forming adjectival/adverbial stems). The exact historical derivation is uncertain, but the combination conveys an intense, sudden manner.