Provenance
Where it comes from.
'Back,' the body part, comes straight from Old English bæc, from a Proto-Germanic root with no certain relatives outside the Germanic languages, one of the oldest and plainest words in English. Many neighboring languages later replaced their equivalents with words meaning 'ridge' or 'spine,' but English kept the short, blunt original.
'Mid' is the equally old English element for the middle, seen in midday, midway, and midnight. Together they mark not the whole back but its center: the specific stretch the name is careful to name.