背く
そむく · somuku · JLPT N1 · verb-godan
to go against
背 = back (of the body); 背く = turn your back on, so go against
The 背 in 背く is the same character as 背中 (back of the body). Turning your back on a rule or a person means going against them, which is the core sense of そむく.
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.