Happen showed up in the late fourteenth century as happenen, from hap, a word that was much more common then and way less common now. Hap showed up in the thirteenth century, meaning chance, or someone’s fortune or fate—happen originally meant something like “to have the fortune to occur”, and then it morphed into just meaning something that occurs, with no good or bad fortune required. Fun fact, before people used happen, Old English used the word gelimpan and Middle English just used befall. As for hap’s origin, it comes from the Old Norse happ and Proto Germanic hap-, from the Proto Indo European kob-, to fit or to succeed. Yeah, there used to be a K there. I guess good luck can help you succeed.
Now, there are several other words with hap in them, some more WTF than others. Mishap, which showed up in the mid thirteenth century, for example. It means bad luck, so with hap meaning good luck (and mis- literally meaning bad). There’s also hapless, which kind of means the same thing. It showed up a bit later than mishap, in the fifteenth century, and the -less means that it’s without good luck. Not sure why they felt they had to distinguish the two, but I guess it was important.
Perhaps showed up in the late fifteenth century as perhappes, though perhap, no S, showed up in the mid fourteenth century. The per means through, and with hap meaning chance, it was saying perchance. But, you know, instead of actually saying perchance. There’s also haphazard, which is a fairly late word, having shown up in the late seventeenth century. It’s just hap (chance) and hazard (danger). There’s a chance of danger. Wow. This makes way too much sense.
And there’s one more word we’re going to look at today. One I’ve actually looked at before, about four years ago. I actually mentioned it being related to happen back then, but never followed up on that. Until now! And that word, of course, is happy. It showed up in the late fourteenth century meaning lucky, and I guess it’s because lucky people are happy? I don’t know, but apparently words that mean happy once having meant lucky is common in several other languages. In any case, the -y means “full of or characterized by”, so with chance (as in, good luck), happy is lucky.
Tony Jebson’s page on the Origins of Old EnglishOld English-English Dictionary