from
The Collaborative International Dictionary of English v.0.48
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.
[1913 Webster]
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.
[1913 Webster]