Source : Webster's Revised Unabridged Dictionary (1913)

Decuman \Dec"u*man\, a. [L. decumanus of the tenth, and by
   metonymy, large, fr. decem ten.]
   Large; chief; -- applied to an extraordinary billow, supposed
   by some to be every tenth in order. [R.] Also used
   substantively. ``Such decuman billows.'' --Gauden. ``The
   baffled decuman.'' --Lowell.