Origin of 楽しみ and 楽しみにする

Question

楽しみ is derived from 楽しむ isn't it? Despite this, it's used as if it were an adjective, and I don't completely understand why it can be used in 楽しみにする. What is the name of the form that 楽しみ is relative to 楽しむ, and why can you use it in にする?

Answer 1

Another set with the same conjugations is 悲{かな}しい・悲しむ・悲しみ　(adjective/verb/noun), where 悲しみ means, more or less, 悲しむこと. In some cases, this noun form of a verb has picked up additional meanings along the way.

楽しみ is something of a special case in that it has at some point acquired both the status of a na-adjective, and a meaning of anticipating that some future event will be 楽しい. Hence, you can say 楽しみに待つ (but probably not 悲しみに待つ).

That said, AをBにする, where both A and B are nouns (and the Aを part may be dropped) is a common grammar pattern with the general idea that A is made to be/treated as/viewed as B.

Answer 2

One meaning of the noun 楽しみ allows for analyzing it as the noun version of the verb 楽しむ. But this doesn't work equally well with the other meaning. If anything, if you accept the notions of i-adjectives and na-adjectives, which seem very common in textbooks for learners of Japanese as a foreign language, this other meaning would be better analyzed as (the stem of) a na-adjective that carries the sense of "looking forward to."

So if you want to explain 楽しみ by using the concepts of i-adjectives, na-adjectives, and derivation (whatever that means), you may say that there are one line of derivation

楽しむ (enjoy) verb -> 楽しみ (enjoyment) noun -> 楽しい (enjoyable) i-adjective

and another line

楽しみだ (having a quality that makes the speaker look forward to it) na-adjective,

where the latter consists of only one grammar class (so it's not really derivation per se).

In short, you can derive a noun and i-adjective from the verb 楽しむ in a straightforward way. Their meanings are as you'd expect from the original verb. But there is another word that looks similar but has a little different meaning, which is the na-adjective 楽しみだ.

I think this explanation is simple enough, goes well with typical grammar-for-learners, and doesn't have many exceptions. An obvious weak point is that there's no equivalent adjective in English that means 楽しみだ as a na-adjective. But it's not my fault. You should blame Enligsh.