浮気
うわき · uwaki · JLPT N1 · noun
matter
浮 = float + 気 = mind: a mind that drifts off from its partner = affair
The same 浮 as 浮かぶ (ukabu, to float). A wandering heart that won't stay with one partner gives both the noun affair and the verb to cheat.
浮 — water radical 氵 + 孚 (phonetic, 'hatch/trust')
浮 is a semantic-phonetic compound. The water radical 氵 gives the meaning 'water,' while 孚 supplies the sound and originally depicted a bird hatching an egg. Together they convey something resting on water, hence 'to float.'