In a manner that is lacking notable events or excitement.
From 'eventless' plus the adverbial suffix '-ly,' which transforms adjectives into adverbs. Standard English word formation created in the 17th-18th centuries.
Adding '-ly' to an adjective is one of English's most productive patterns—you can do this to almost any adjective to create an adverb, which is why English can seem both orderly and creatively endless at once.
Complete word intelligence in one call. Free tier — 50 lookups/day.