เจ้าอยู่หัว

From เจ้า (jâao, “chief, lord, master; god; priest; royal person; etc”) +‎ อยู่ (yùu, “to be at, in, on, etc; to remain, to stay; etc”) +‎ หัว (hǔua, “head”); literally meaning:

(a) "lord above [all/our/my] head[s]",

(b) "lord at [all/our/my] head[s]", or

(c) "[one with] lord on the head".

The suggested meaning (c) is based on the traditional belief that a future Buddha has a figure of Lord Buddha upon his/her head.

เจ้าอยู่หัว • (jâao-yùu-hǔua)

1. (พระ~, สมเด็จพระ~, พระบาทสมเด็จพระ~) an honorific for or term of address to a monarch, archaically applicable to high-ranking royal persons also.

• In modern usage, the term is preceded by พระบาทสมเด็จพระ (prá-bàat-sǒm-dèt-prá) when applied to a monarch who has been formally crowned, and by สมเด็จพระ (sǒm-dèt-prá) when applied to a monarch that has not or not yet been formally crowned.