plerumque

Plerumque is the neuter accusative singular of plerusque, "the greater part", frozen into an adverb: an accusative of extent doing the work of "for the most part". It sits between saepe, which counts occasions, and fere, which hedges an estimate. Plerumque claims a majority.

Two things to keep straight. The -que on the end is the generalising enclitic you also meet in quisque and uterque, not the conjunction "and", so it never joins anything. And the same adjective is alive and well in the plural: plerique, pleraeque, pleraque, "most people, most things", which declines normally and takes a partitive genitive. Write it as one word; "plerum que" is a manuscript accident, and so is the occasional plaerumque.