背中
せなか · senaka · JLPT N4 · noun
back
背中 = back + middle: se (back) + naka (middle), the middle of your back
背 (se) means back or stature, and 中 (naka) means middle or inside. Together they name the central flat of the back. You already know naka from 中 in words like 中 (inside) and 田中 (Tanaka), so only se is new.
背 — north 北 (phonetic) + flesh 月/肉
Originally a phono-semantic compound: 月 (a form of 肉, 'flesh/body') gives the meaning 'back of the body,' while 北 supplies the sound. Interestingly, 北 itself depicted two people back-to-back and once meant 'back'; when it was borrowed for 'north,' 背 was created with the flesh radical to keep the original sense.
中 — vertical stroke 丨 through enclosure 口
Pictograph of a banner or pole piercing the center of a target or territory, depicting the idea of 'middle' or 'center'. The horizontal frame represents boundaries, and the vertical line marks the central axis. The meaning extended from spatial center to 'middle, central, China' (中国, the Middle Kingdom).