To betide is to happen to somebody, in old use. Woe betide you, whatever may betide, betide him ill.
It survives almost entirely in woe betide, which is a warning rather than a curse — woe betide anybody who tries. The tide in it is the old sense of time or occasion, the same one in tidings and in eventide. Note that the be- makes a verb, as in befall.
1verbbetideTo happen to somebody.
“Woe betide anyone who wakes the baby after I've finally got her to sleep!”
betide is a happening.
Middle English bityden is be- plus tide, in the old sense of happening. Tidings and betide come from the same happening.
Proto-Indo-European
*deh₂- — to divide, share
Old English
tīdan — to befall, betide, happen
Middle English
bityden
English, today
betide
Built from
From Middle English bitydenbetide, betiden, betyd, betyde, bitide, bitiden, bytide, bytyden; from bi- + tyden (“to come about, happen, occur; to befall, become of, happen to (someone); to be the fate of (someone); to await (someone); to fare, get along”); tyden is derived from Old English tīdan (“to befall, betide, happen”), related to tīd (“time; season; hour”) (both ultimately from Proto-Indo-European deh₂- (“to divide, share”) or its extended form deh₂y-, whence déh₂itis (“time”)) + -an. The English word is analysable as be- + tide.
Etymology from Wiktionary, CC BY-SA