To gush forth suddenly; to rush out with force and enthusiasm (dialectal or archaic).
Possibly from a- (prefix) + gush, or a variant of 'gush' with prefix. The exact origin is uncertain, but it follows English patterns of intensifying verbs with prefixes.
Dialectal verbs like 'agush' often disappear from standard English, but they survive in regional speech and literature. They're like fossils of earlier ways people spoke, preserved in corners of the language.
Complete word intelligence in one call. Free tier — 50 lookups/day.