The Collaborative International Dictionary of English v.0.48:

Sesquitertian \Ses`qui*ter"tian\, Sesquitertianal
\Ses`qui*ter"tian*al\, a. [Sesqui- + L. tertianus belonging to
   the third. Cf. Tertian.] (Math.)
   Having the ratio of one and one third to one (as 4 : 3).
   [1913 Webster]