godan verb (-su)
start barking; bark out
Describes a dog or similar animal suddenly beginning to bark or barking loudly. The gloss 'give mouth' is a hunting term for a hound baying; this nuance is rare and domain-specific.
夜中に隣の犬が急に吠え出した。
The neighbor's dog suddenly started barking in the middle of the night.
猟犬が獲物の匂いを
The hunting dog caught the scent of prey and gave mouth.