Lewis & Short

sesquĭ (sesque), num. adv. [perh. contr. from semis-qui], one half more, more by a half. As a separate word it occurs only once: ut necesse sit partem pedis aut aequalem alteri parti aut altero tanto aut sesqui esse majorem (just after, sesquiplex), Cic. Or. 56, 188. But freq. joined in one word with designations of number or quantity, with the signif. of once and a half. Joined with numerals (octavus and tertius), like the Gr. ἐπί (in ἐπόγδοος, ἐπίτριτος, etc.), it denotes an integer and such a fraction over as the numeral designates; v. sesquioctavus, etc.