Woe betide you
A prediction, usually expressed as a warning following someone's bad behaviour, that you may suffer future misfortune.
'Woe betide you' has a rather archaic feel. 'Betide' is hardly ever used now outside this expression and 'woe' is generally reserved for hamming it up in 'Ye Olde' B features. It is also used in the expression 'woe is me', which is itself venerable and has a strong claim to be the earliest expression that has migrated from another language into English. UK readers of a certain age will no doubt remember the much missed Frankie Howerd's 'woe, woe and thrice woe' catchphrase. A Google search for "woe betide you" does get 600,000+ hits though, so someone must still be using it.
'Woe betide me' was a common early precursor and appears in William Langland's Middle English narrative poem The vision of William concerning Piers Plowman, 1393:
Er ich wedde suche a wif· wo me by-tyde [If I marry such a wife, woe betide me]
We now only use the word 'tide' to denote the regular rising and falling of the sea. We can get a better understanding of what 'tide' and 'betide' mean by substituting 'tide' with 'time', which is just what the mediaeval clerics did - the two words were near enough synonymous. Knowing that 'tide' means 'period of time' or 'season', we can see that a lunar tide can be translated as 'a period of approximately twelve and a half hours' and 'woe betide you' as 'you are in for a bad time'.
The tide/time transliteration also survives in 'good tidings', i.e. 'a good time', 'tide over', i.e. 'make last for a time' and in the names of festivals like Whitsuntide. We can also shorten the reduplicated phrase 'time and tide' if we choose, as one word just repeats the other.
See also: reduplicated phrases.