Exact historical dictionary sense
Often in Webster's 1913 dictionary
The 1913 record labels often as adverb. Its source-bounded etymology and selected historical definition appear below.
WEBSTER 1913adverb
Often
[Formerly also ofte, fr. oft. See Oft., adv.]
Cross-verified historical senseHeadwordoften
Part of speechadverb
Historical etymology[Formerly also ofte, fr. oft. See Oft., adv.]
Selected historical definitionFrequently; many times; not seldom.
Signed record IDwebster1913-often-adverb-9fe6331af0e1
Source locatorCIDE.O, paragraph 1510; Gutenberg eBook 665
Quick answer
In Webster's Revised Unabridged Dictionary (1913), "often" is labeled adverb. Its etymology is recorded as: "[Formerly also ofte, fr. oft. See Oft., adv.]" The selected historical definition says: "Frequently; many times; not seldom."