Where did the phrase "used to" come from?
Why does "used to" mean "accustomed to"?
Why is "used to" used to indicate a recurring past event? In
I used to be used to using it.
there are three meanings of "use". I ask about the etymologies of the two bolded meanings.
Each of the three meanings can be paraphrased...
I [was in the habit of being] [accustomed to] [employing] it.
I assume OP wonders about the first meaning, but in reality I think it's just a tautological overlap with the second. It makes for an ugly sentence, to say the least.
The association of used with acclimatised over time, through repeated exposure or use seems unremarkable to me. I imagine the usage could have been re-coined repeatedly before it became a familiar part of normal speech and writing.
The expression used to in the sense of was in the habit of, has been around a very long time, as @Philoto assiduously researched. But originally it was as likely to be the present tense form use to as past tense used to. I think any such present tense usage today is simply by mistake, not in an attempt to convey 'archaic' connotations.
For some reason I can't really explain, the past tense form shot to prominence in the early 1800s.
AllExperts.com:
According to the Oxford English Dictionary, the earliest text using "used to" in the context you ask about is Robert Mannyngs "Handlyng Synne 1303." The quote cited is "For ryche men vse comunly Sweryn grete othys grysly." Translated: "For rich men used to commonly swear great, grisley oaths."
It was in "very common use" from around 1400 onward, but today only appears in the past form of "used to." "wont to do" is another archaic expression that carried the meaning of "used to" in reference to habitual activity in the past.