Source : Webster's Revised Unabridged Dictionary (1913)

Lowermost \Low"er*most`\, a. [Irreg. superl. of {Low}. Cf.
   {Uppermost}, {Foremost}, etc.]
   Lowest.

Source : WordNet®

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