What is the etymology of the expression 'used to' in the senses ...

Welcome, kofga. I'm sorry no-one has gotten back to you within a couple of days. I suspect it's because you've got everyone stumped. Good for you!

I am a former member who noticed your unanswered question and I can't resist the challenge you have presented.

I checked Wordsmith's Archives for "used to" and, although there are many entries there, none fits the bill.

Looking at your own personal background, I'll bet you have some personal theories regarding the etymology of "used to" in the 2 senses you have described. Care to share them with us?

The best explanation I can suggest personally is that "used to" is a logical extrapolation of the verb "to use". When we "use" something, it becomes "used". What is "used" is often discarded or used no more. Hence, "I used to go bungee jumping" [but I don't anymore].

Similarly, when we use something frequently, we "get used" to it. It becomes familiar and natural, even automatic as any skill becomes through repetitive practice.

Some people are all thumbs when it comes to sports, however. Hence, "I never got used to bungee jumping."

Actually, bungee jumping isn't the best example, is it, because bungee jumping isn't really a skill. Someone who "never gets used to" bungee jumping, never gets used to the scare. They can't overcome their fear of the jump, no matter how many times they try it. Same thing with the fear of flying, for some folks.

Thanks for your challenging question, kofga. Hope you will share your own theories.