What part of speech is “worth”? [duplicate]

Solution 1:

It is actually an adjective. The OED has a note about it:

Almost always (now only) in predicative use, or following the sb. as part of a qualifying phrase.

I can see why you would think of it as a preposition, in that it seems to take an object.

• That cow is worth five hundred dollars to me.

When used following its “object”, it acts as a noun with a possessive in front of it — or you might think of it as a postposition that takes a genitive.

• I’d like ten dollars’ worth, please.

It’s a very old word. The OED’s second citation for it is from the 7^th century:

• A. 695 Laws Ine lviii, ― Oxan horn bið x. pæninga weorð

Which (I presume) means something like “Oxen horn be-eth 10 pennies worth”. By the time Old English had creoled into Middle English, we see the now-familiar formula:

• C. 1350 Athelston 391 ― Now is my goode hors forlorn,··He was wurþ an hundryd pounde.

If someone asked how much something was valued, you would think of valued as an adjective. They can similarly ask how much something is worth, so it stands to reason that worth is also an adjective. But yes, it is weird.

One upon a time there was an adjective wurthe, meaning worthy, which eventually merged with worth. The OED writes:

OE. wierðe, wyrðe, etc., a derivative from weorþ worth sb.1 or a. In OE. and early southern ME. texts distinguishable from worth a., but subsequently merged with it.

The most legible citation is this:

• C. 1325 Chron. Eng. 741 in Ritson Metr. Rom. II. 301 ― Afterward, ase he wes wurthe,··An abbot him remue wolde.

Wurthe also meant merited or deserved, as in this citation:

• A. 1225 Ancr. R. 138 ― We moten þauh don him wo, ase hit is ofte wel wurðe.

It is not surprising that it merged with worth.

A more clearly adjectival use is the now archaic:

• 1871 B. Taylor Faust II. ii. ii. 148 ― Little worth is woman’s beauty, So oft an image dumb we see.

Solution 2:

I have seen some authorities (including the CGEL) put worth in a class of words called marginal prepositions. These words can be used mostly like prepositions, but "have affinities with other word classes"; usually they come from other parts of speech that have undergone some grammaticalization. This characterization is admittedly vague, but the number of words in the class is small, and it has been observed that "Their occurrence is tightly constrained by grammatical and lexical factors." (Rankin and Schiftner, 2009)

Other marginal prepositions in English include:

following, concerning, considering, excluding, given, granted, pending, worth, minus, plus, instead of

Other languages sometimes have marginal prepositions too. Generally they seem to be words that are derived from other parts of speech and are in the process of becoming regular prepositions.