Encara no tenim significats per a "add authoritatively".
1And go to bed early to-night, he added authoritatively.
2"The steward told me so," he added authoritatively.
3"Deacon," he added authoritatively, "arrest that foreigner."
4"If I'd only remembered about you," he murmured, and turning heavily in his chair, he added authoritatively: "Now tell me about it.