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.
The dog came at me, barking fiercely.
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.