From the second edition (1989):
-ty, suffix1
denoting quality or condition, representing ME. -tie, -tee, -te (early ME. -teð), from OF. -te (mod.F. -té), earlier -tet (-ted):—L. -itātem, nom. -itās. Such Latin types as bonitātem, feritātem, were in OF. normally reduced to two syllables (bontet, fertet) by elision of the -i- between the two stresses, so that -tet, later -te, became the regular form of the suffix. The final dental still appears in some early adoptions in ME., as plenteð, plenteth plenty (c 1250, in use till c 1600), and is characteristic of the Scottish forms bountith, daintith, and poortith (q.v.). The reduced form -te, however, is found in words recorded from shortly before or after 1200, such as bonte bounty, cruelte cruelty, debonerte debonairness, deinte dainty (n.), plente plenty, poverte poverty, purte purity, and vilte vileness. Among others which appear somewhat later are certeynte certainty, Cristente Christenty, freelte frailty, novelte novelty, and sotelte subtlety. Varying forms of the stem are found in the words now or formerly represented by beauty, fealty, lealty, †lewty, loyalty, †realty, †rialty, and royalty. From the types lealte, realte, the ending -alte (mod.F. -auté) was in OF. extended to formations from different stems, and many words of this form (ultimately written with -alty) established themselves in English, as admiralty, casualty, commonalty, †generalty, mayoralty, †principalty, †regalty, severalty, specialty, spiritualty, temporalty. Most of these date from the 14th or early 15th century; penalty appears to be of later introduction (1512). An obsolete type of formation is exhibited by curiouste, hid(e)ouste, and joyouste. In OF. certain analogies led to the frequent substitution of -ete for -te, but this form of the suffix is only occasionally adopted in English, as in the obsolete noblete, purete, and simplete; the early sauvete is now represented by safety. Under Latin influence many words in OF. also appear with -ite (mod.F. -ité) in place of -(e)te; hence English forms in -ity, which in many cases (as in F.) have supplanted those in -ty.

Although occurring in a large number of words the suffix has shown little productive power in English; evelte, everlastingte, and overte occur in the 14–15th cent., and shrievalty, sheriffalty, have had currency from the beginning of the 16th cent., but such formations are very rare.
Such words as faculty, difficulty, honesty, modesty, puberty, represent Latin formations in which the suffix -tās is directly added to a consonantal stem. The number of these in English, as in French, is very small.

The early form of the suffix (-te, or -tee) remained in use down to the 16th cent., but from the 15th was gradually supplanted by -tie, -tye, and the surviving -ty.