From Longman Dictionary of Contemporary Englishbetidebe‧tide /bɪˈtaɪd/ verb → woe betide somebody→ See Verb table
Examples from the Corpus
betide• But woe betide the culprit who trespasses on the wrong side of 50.• She has a sharp cutting edge and woe betide the Europhile who treads on her toes.• But woe betide the tight-fisted villager who tries to take advantage.• But woe betide you if you make your splash with a financial killing or by getting a huge advance.Origin betide (1100-1200) tide “to happen” ((11-19 centuries)), from Old English tidan 