IPA: //ˈbæk.eɪk//
KK: /ˈbækˌeɪk/
A feeling of pain or discomfort in the back, particularly in the lower part.
After lifting the heavy box, I developed a backache that lasted for days.
Backache → It is formed from "back" (from Old English "bæce", meaning the rear part of the body) and "ache" (from Old English "æce", meaning pain). The word "backache" refers to pain located in the back area of the body.
Think of the word 'back' as the part of the body where the pain is felt, and 'ache' means pain. So, backache is simply pain in the back.