costly以ly结尾为什么是形容词？

costly以ly结尾为什么是形容词？

形容词+ly：副词

名词+ly：形容词

suffix forming adjectives from nouns and meaning "having qualities of, appropriate to, fitting;" irregularly descended from Old English -lic, from P.Gmc. *-liko- (cf. Old Frisian -lik, Dutch -lijk, Old High German -lih, German -lich, Old Norse -ligr), related to *likom- "appearance, form" (cf. Old English lich "corpse, body;" see lich, which is a cognate; cf. also like (adj.), with which it is identical).

adverbial suffix, Middle English, from Old English -lice, from P.Gmc. *-liko- (cf. Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).

在这里cost作为名词，意为“价格、成本、费用”，加上后缀ly，变为costly，意为“昂贵的，代价高的”，这时候符合第二条规则。

因此costly为形容词。另外，它还有一个形容词形式：

costless 不须花钱的,无须付出代价的。