副词后缀-s几乎是现代英语所有名词后缀的复数形式，这个后缀是从古英语-as 逐渐地扩充到中古英语里面。这里只是简单介绍了后缀-s来源，实际上，英语单词很在一部分单词的来源比较曲折，而且来源于不同的语言，这就像是中国普通话和各地方言一样，整个欧洲如果看成一个国家的话，就非常像中国的变通与地方音的千差万别了，当然，欧洲大部分语言本为一家，后来不断的演变，才有了现在的国家密集，语言复杂。为了让大家更好的理解，后缀-s的来源，这里给出纯英文词源，供大家参考。

英文词源：

-s (1)
suffix forming almost all Modern English plural nouns, gradually extended in Middle English from Old English -as, the nominative plural and accusative plural ending of certain "strong" masculine nouns (such as dæg "day," nominative/accusative plural dagas "days"). The commonest Germanic declension, traceable back to the original PIE inflection system, it is also the source of the Dutch -s plurals and (by rhotacism) Scandinavian -r plurals (such as Swedish dagar).

Much more uniform today than originally; Old English also had a numerous category of "weak" nouns that formed their plurals in -an, and other strong nouns that formed plurals with -u. Quirk and Wrenn, in their Old English grammar, estimate that 45 percent of the nouns a student will encounter will be masculine, nearly four-fifths of them with genitive singular -es and nominative/accusative plural in -as. Less than half, but still the largest chunk.

The triumphs of -'s possessives and -s plurals represent common patterns in language: using only a handful of suffixes to do many jobs (such as -ing), and the most common variant squeezing out the competition. To further muddy the waters, it's been extended in slang since 1936 to singulars (such as ducks, sweets, babes) as an affectionate or diminutive suffix.

Old English single-syllable collectives (sheep, folk) as well as weights, measures, and units of time did not use -s. The use of it in these cases began in Middle English, but the older custom is preserved in many traditional dialects (ten pound of butter; more than seven year ago; etc.).

-s (2)
third person singular present indicative suffix of verbs, it represents Old English -es, -as, which began to replace -eð in Northumbrian 10c., and gradually spread south until by Shakespeare's time it had emerged from colloquialism and -eth began to be limited to more dignified speeches.

同缀词：

1.afternoons
adv.每天下午

2.nights
adv.每夜，在夜间

3.weekends
adv.在每个周末

4.sometimes
adv.有时

5.besides
adv.此外，而且

6.unawares
adv.不知不觉地

7.nowadays
adv.现今，当今

8.outdoors
adv.在户外

9.indoors
adv.在屋内

10.upstairs
adv.在楼上，往楼上

11.downstairs
adv.在楼下，往楼下

12.weekdays
adv.在每个周日