You're right that it's not 민자. It should be 민짜. - 민자 (= 민간 자본) = private(non-public) funds/capital. - 민짜 = a simple/plain item. It describes something very basic with no fashionable design or ornamentation. It is used most often for clothes. Describing a body part like 겨드랑이(armpit) as 민짜 is not common. I think they're saying that its flat shaped or hairless, or looks very simple in some other way.

I think 'armpit doesn’t have any hair'