The Collaborative International Dictionary of English v.0.48:

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.