ใต้ฝ่าละอองพระบาท

Etymology
🇰🇲; literally "the underneath of the particles at the soles of (your) holy feet".

Noun

 * 1) used as a form of address to royal persons.

Usage notes
Now conventionally applied to kings consort, queens consort, king fathers, queen mothers, crown princes, crown princesses, Princesses Royal, and the like.