What is the etymology of the expression 'used to' in the senses which convey a)having done something at one time in the past (I used to go bungee jumping) and becoming accustomed to something (I never got used to bungee jumping)? Thanks

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.

I think that may have been discussed here one time in the distant past*, kofga, but I for one am unwilling to take the time to Search for it, because there are bound to be hundreds (at least) of times it was, um, used.
You might try doing an on-line search for "word and phrase origins", then going to the sites that come up. Or you can save yourself some time and go to the Useful language links thread in I & A and click on Max's site (posted by AnnaStrophic).
*It goes without saying that I don't remember what the discussion consisted of, if it did happen.

use is one of those words which historically has been used to mean a whole lot of different things -- OED2 lists 25.. no, 26 senses. only a few of these are intransitive, e.g.:

20. To do a thing customarily; to be in the habit of so acting or doing; to be wont to do. (Chiefly in clauses introduced by as, and now only literary.)
[first attested in 1380] 1875 BROWNING Aristoph. Apol. 365 Die at good old age as grand men use.

which leads to:
21. With to and inf.: To be accustomed or wont to do something.
In very frequent use from c 1400, but now only in pa. tense used to, with pronunc. (just tu, just), and colloq. in did (not) use (or used) to: see also USEN'T, USETER; used to could.
[attested to Chaucer in 1385] 1670 MILTON Hist. Eng. VI. 304 The English then useing to let grow on their upper-lip large Mustachio's. 1728 GAY Begg. Op. II. iv, You are not so fond of me, Jenny, as you use [sic] to be. 1767 Woman of Fashion II. 26 How did we all use to admire her! 1837 LOCKHART Scott I. iv. 122 He used to get all the copies of these ballads he could. 1873 C. M. YONGE Pillars of House II. xvi. 105 Did Alda use to be nice, or is it love? 1884 W. C. SMITH Kildrostan 53 You used to be a leal, true-hearted girl. 1925 S. LEWIS Arrowsmith xviii. 192 Didn't we used to have fun. 1927 E. HEMINGWAY Men without Women 154 He certainly did used to make the fellows he fought hate boxing. 1935 E. FARJEON Nursery in Nineties III. i. 124 Mama, did you use to be a flirt? 1963 V. NABOKOV Gift ii. 117 And now I continually ask myself what did he use to think about in the solitary night. 1974 Radio Times 28 Feb. 25, I suppose I did use to be a prophet of doom.

of course, the OED being historically descriptive, this all says nothing about the "correctness" of using "used to" vs. "use to"..