Webster’s Revised Unabridged Dictionary · 1913
Decuman
Dec′u·man (dĕk′ū·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.