Source : Webster's Revised Unabridged Dictionary (1913)

Sesqui- \Ses`qui-\ [L., one half more, one and a half.] (Chem.)
   A combining form (also used adjectively) denoting that three
   atoms or equivalents of the substance to the name of which it
   is prefixed are combined with two of some other element or
   radical; as, sesquibromide, sesquicarbonate, sesquichloride,
   sesquioxide.

   Note: Sesquidupli- is sometimes, but rarely, used in the same
         manner to denote the proportions of two and a half to
         one, or rather of five to two.