I grew up saying oxter and as an adult always wondered where it came from. I learned a couple of weeks ago that armpit in Dutch and Afrikaans is oksel - only other languages that I have heard something similar!
Oxter: The armpit. From the Old English oxta or ohsta. The word oxter is used in certain areas of the world (Scotland, Ireland, Northern England), reminding us that there are many local and colloquial names for parts of the human anatomy.
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