So, my mom's weird and when my brothers and I were little
she would call our armpits 'bird nests' and every time we'd have our arms up she'd tickle us and say 'don't let those eggies fall out' because I don't know she's weird.
Anyway, in 2nd grade I don't even remember what was going on but my teacher asked 'what's this called?' pointing to her armpit. Me, being the proud second grader I was, I raised my hand and she called on me and I said, 'your bird nest.'
She was quiet for a long time before callng on someone else who answered 'armpit'
And that is how I learned what your armpit is called.