Wiktionary
smooth as a baby's bottom
a. (context simile English) Extremely smooth.
Usage examples of "smooth as a baby's bottom".
Left her smooth as a baby's bottom, apart from the burns, that is.
He was a little shorter than me, with a pale, creamy complexion and rosy red cheeks that were as smooth as a baby's bottom.
She took my hand and slipped it up her thighs smooth as a baby's bottom up and up to her panties but there were no panties and the tangle of welcome stuff was like an oasis and I flashed on how once in Morocco, south of the Atlas in the high Sahara, Berry and I had chanced upon an oasis called Source Bleu du Meski, all palms and shade and blue water that tasted of copper.
You all seem to have faces ``as smooth as a baby's bottom'', I must say!