From the second edition (1989):
[ME. -ite, a. F. -ité, L. -itāt-em]
the usual form in which the suffix (L. -tās, -tātem, expressing state or condition) appears, the i- being orig. either the stem vowel of the radical (e.g. L. suāvi-tās suavity), or its weakened repr. (e.g. L. puro-, pūri-tās purity), rarely a mere connective (e.g. L. auctōr-i-tās authority; so ME. emperorite, in Vernon MS., St. Ambrose 886). The last became more frequent in med. and mod.L., and the mod. langs., in abstracts from comparatives, as majority, minority, superiority, inferiority, interiority. Hence such formations as egoity, with playful or pedantic nonce-words of Eng. formation, as between-ity, coxcomb-ity, cuppe-ity, table-ity, threadbar-ity, woman-ity (after humani-ty), youthfull-ity.
After i, -ity becomes -ety, as in pie-ty, varie-ty (L. pietātem, varie-tātem). The termination was in L. often added to another adj. suffix, e.g. -āci-, -āli-, -āno-, -āri-, -ārio-, -bili-, -eo-, -idi-, -ido-, -ili-, -īli-, -ino-, -īno-, -io-, -īvo-, -ōci-, -ōso-, -ui-, -uo-, etc., whence the Eng. endings -acity, -ality, -anity, -arity, -ariety, -bility, -eity, -idity, -ility, -inity, -iety, -ivity, -ocity, -osity, -uity, some of which, as -bility (-ability, -ibility) attain almost to the rank of independent suffixes. The earlier popular Fr. form was -eté, in Eng. -ety and -ty, as in safety, bounty, plenty: see -ty.