Etymology From Middle English happenen, hapnen, augmented from Middle English happen (“to come to pass, happen”), perhaps from Old English hæppan (“to move accidentally, slip”) and/or from Old Norse *happa, *heppa, from Proto-Germanic *hampijaną (“to fit in, be fitting”). Equivalent to hap (“a chance, occurrence, byfall”) + -en (verbal suffix).
1
To occur or take place.
Let me tell you how it happened.
Seeing the British establishment struggle with the financial sector is like watching an alcoholic who still resists the idea that something drastic needs to happen for him to turn his life around.2013 June 28, Joris Luyendijk, “Our banks are out of control”, in The Guardian Weekly, volume 189, number 3, page 21:
'By my soul! I believe something bad has happened me,' he muttered, and popped up his window, and looked out, half dreaming over the church-yard on the park beyond […]1863, Sheridan Le Fanu, The House by the Churchyard:
3
With infinitive To do or occur by chance or unexpectedly.
Take an umbrella in case it happens to rain.
Do you happen to have an umbrella?
2 more examples
If you should (happen to) need any help, I'll be there.
It happened that a police car was passing my house just at that moment.
4
Followed by on or upon To encounter by chance.
Unexpectedly, in a nook close by the farmhouse, he happened upon a spot where the vintage had actually commenced.1860, Nathaniel Hawthorne, chapter 30, in The Marble Faun:
Etymology From Middle English happenen, hapnen, augmented from Middle English happen (“to come to pass, happen”), perhaps from Old English hæppan (“to move accidentally, slip”) and/or from Old Norse *happa, *heppa, from Proto-Germanic *hampijaną (“to fit in, be fitting”). Equivalent to hap (“a chance, occurrence, byfall”) + -en (verbal suffix).
Yeah, well happen if he'd gone quietly, she wouldn't have needed to show him who's in charge.2016, Sally Wainwright, Happy Valley, spoken by Catherine Cawood (Sarah Lancashire), Manchester: Red Production Company; distributed by BBC Studios, published 2016: