expression
giving a royal welcome; extending every courtesy
Fixed phrase used to describe treating a guest with the utmost hospitality and respect. Often appears as 下にも置かぬもてなし or similar.
彼は下にも置かぬもてなしを受けた。
He received a royal welcome.
社長は来客を下にも置かぬ扱いで迎えた。
The company president welcomed the visitor with every courtesy.
手厚い is an adjective meaning 'hospitable' or 'cordial', while 下にも置かぬ is a set phrase describing the manner of treatment, often modifying nouns like もてなし.
Literally 'not even placing (someone) down', implying that the guest is treated so well they are never set aside. The exact historical derivation is uncertain, but the phrase is conventionally associated with extreme hospitality.