Why is ホームシック a noun, not a な-adjective?


In class, I was told that ホームシック is a noun, not a な-adjective.

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    ホームシックの時、何をしますか。
    
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By contrast, the closest English word to "ホームシック", which is "homesick", is an
adjective.

Why is ホームシック a noun? Is it describing a disease (if you had to use English,
you might say "homesickness"), rather than the state of having the disease?




First of all, it's worth noting that Japanese has no 形容詞 or 形容動詞(な-adjective)
which directly corresponds to the English adjective _sick_. (although you can
say 「彼の具合【ぐあい】が悪【わる】い 」, if you don't mind replacing the subject)

We can say 「彼 は [病名] だ」、「[病名] の 人」、「 [病名] に なる」、where [病名] can be 癌 (cancer),
肺炎 (pneumonia), 糖尿病 (diabetes), 骨粗鬆症 (osteoporosis), or 病気 (disease) in
general. And they are all nouns. We usually cannot say 「 [病名] な 人」, so 「癌な人」
or 「糖尿病な人」 is wrong.

I think that's why we often treat the loanword ホームシック also as a noun. Words
such as _lovesick_ or _airsick_ have no corresponding adjectives either in
Japanese; instead we have nouns 恋煩【こいわずら】い (love sickness) or 飛行機酔【ひこうきよ】い
(air sickness).

That said, I think this rule is somewhat loosened if certain 外来語 diseases are
concerned. In fact, **both 「ホームシックな時」 and 「メタボな人」 sound as natural as
「ホームシックの時」 and 「メタボの人」 to me**. Japanese tend to create many な-adjectives from
English adjectives, as in スマートな, リスキーな, etc. They can even create な-adjectives
from English nouns, such as
[ワンパターンな](http://en.wiktionary.org/wiki/%E3%83%AF%E3%83%B3%E3%83%91%E3%82%BF%E3%83%BC%E3%83%B3),
ハイリスクな. I see no reason why _homesick_ cannot act as a な-adjective as well as
a noun.

Lastly, some people may argue that Japanese young people use "病気な人", or more
commonly, "ビョーキな人". Such use of 病気 is a slang and [has a special
meaning](http://en.wiktionary.org/wiki/%E7%97%85%E6%B0%97) (abnormal, crazy,
mad). Use "病気の人" for someone with a real medical disorder.

