expression
treat with the utmost hospitality; give a royal welcome
Idiomatic expression meaning to treat someone with extreme care and respect, as if they were too precious to be placed even on the floor. Often used in the form 下にも置かないもてなし or 下にも置かない扱い.
彼は来客を下にも置かないもてなしで迎えた。
He welcomed the guest with the utmost hospitality.
祖母は孫を下にも置かないほど可愛がっている。
The grandmother dotes on her grandchild as if they were the most precious thing in the world.
手厚い means 'warm and generous' in treatment or hospitality, while 下にも置かない is a more vivid, idiomatic expression emphasizing extreme care and respect.
Literally 'not even placed on the floor', suggesting that the person is treated so preciously that they are not set down on the ground. The exact origin is uncertain, but it is a fixed idiomatic expression.