sesqui-

1.
a combining form meaning “one and a half,” used in the formation of compound words:
sesquicentennial.
Origin
< Latin sēsqui- < *sēm(i)sque, equivalent to sēmis half-unit, semis + -que and (cognate with Greek té, Sanskrit ca, Gothic -uh)
British Dictionary definitions for sesqui-

sesqui-

prefix
1.
indicating one and a half: sesquicentennial
2.
(in a chemical compound) indicating a ratio of two to three: sesquioxide
Word Origin
from Latin, contraction of semi- + asas² + -que and
Word Origin and History for sesqui-

word-forming element, from Latin sesqui- "one-half more," contraction of *semis-que- "a half in addition," from semis "a half" (see semi-) + -que "and," from PIE *kwe-.