Bottom
.At bottom. Radically, fundamentally: as, the young prodigal lived a riotous life, but was good at bottom, or below the surface.
“If one of the parties … be content to forgive from the bottom of his heart all that the other hath trespassed against him.”—Common Prayer Book.
“My ventures are not in one bottom trusted.”—Shakespeare: Merchant of Venice, i. 1.
To have no bottom. To be unfathomable.
To touch bottom. To reach the lowest depth.