RhymeZone: rethe definitions

Definitions of rethe:

adjective: (Now chiefly dialectal) (of persons) Fierce; cruel; savage; stern; zealous; ardent; keen.
adjective: (Now chiefly dialectal) (of things) Terrible; dreadful; severe.
adverb: (Now chiefly dialectal) Furiously; violently; wildly.

(Definitions from Wiktionary)