咳
せき · seki · JLPT N3 · noun
cough
🫁 咳 = 口 mouth + 亥; seki sounds like a sec: a cough cuts you off for a sec
The 口 (mouth) radical shows the sound comes from the mouth, and 亥 gives the old reading. For せき, think of a cough that stops your sentence for a sec, then you carry on talking.
咳 — mouth 口 + child 亥
A phono-semantic compound: 口 (mouth) gives the meaning, since coughing comes from the mouth, while 亥 supplies the sound. Originally the character could mean a child's laugh (read hāi), but its dominant modern sense is 'to cough' (ké).