Source : Webster's Revised Unabridged Dictionary (1913)

Nethermost \Neth"er*most`\, a. [AS. ni(?)emest. See {Nether},
   and cf. {Aftermost}.]
   Lowest; as, the nethermost abyss. --Milton.

Source : WordNet®

nethermost
     adj : farthest down; "bottommost shelf" [syn: {bottommost}, {lowermost}]