In a manner that relates to or involves making an official decree or authoritative decision.
From decretive + -ly (adverb suffix). The -ly transforms the adjective into an adverb describing how something is done in a decreeing manner.
While extremely rare, this word would describe acting with the absolute authority of a ruler declaring law—speaking decretively means speaking as if your words are law.
Complete word intelligence in one call. Free tier — 50 lookups/day.