IPA: /ˈɑːrmbænd/
KK: /ˈɑrmˌbænd/
A band that is worn around the upper arm, usually for identification, decoration, or as a sign of mourning or protest.
She wore a black armband to show her respect for the deceased.
Armband is formed from "arm" (from Old English "earm", meaning the upper limb) and "band" (from Old English "banda", meaning a strip or ribbon). The word "armband" refers to a band or strip worn around the arm.
Think of a 'strip' or 'ribbon' ('band') that you wear around your 'arm' — that's what an armband is.