ใต้ฝ่าพระบาท

Etymology
🇰🇲; literally "the underneath of the soles of your holy feet".

Noun

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

Usage notes
Now conventionally used to address:
 * high-ranking royal persons, especially princes or princesses holding the title or ;
 * high-ranking priests, especially princes ordained as Buddhist monks and holding the title.