In a tedious, dull, or uninteresting manner that causes one to lose attention or interest.
From 'boring' (present participle of 'bore', meaning to make tedious) plus the adverbial suffix '-ly', allowing the adjective to modify verbs and other adjectives.
The word 'boring' only became common in the 1700s—before that, English speakers would say something was 'wearisome' or 'tedious,' but 'boring' perfectly captured the image of drilling endlessly into someone's patience.
Complete word intelligence in one call. Free tier — 50 lookups/day.