To thank thoroughly or thank very much, an archaic or obsolete form of expressing gratitude.
Old English 'bethancan' from 'be-' (thoroughly) plus 'thancan' (to thank), following the medieval English pattern of using 'be-' to intensify verbs.
Words like 'bethank' show that Old English speakers had a whole toolkit of prefixes to modify verbs, creating nuance—'be-' could mean 'thoroughly,' 'around,' or 'cover completely,' giving speakers precise expressiveness we've mostly lost.
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