Merriam-Webster Collegiate Dictionary - sesqui-

 combining form  Etymology: Latin, one and a half, half again, literally, and a half, from semis half of an as, one half (probably from semi- + as as) + -que (enclitic) and; akin to Greek te and, Sanskrit ca, Gothic -h, -uh  1. one and a half times sesquicentennial  2. containing half again as many atoms sesquiterpene  3. intermediate ; combination sesquicarbonate