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Consider the following sentence:

> その薬は去年期限切れ **の** だったので、捨てました。

I'm trying to explain what the bolded の does in the above sentence.

This is what I came up with:

> の is used as a back referral
>
> The sentence would look like this :
>
> その薬は去年期限切れ **の** (薬)だったので、捨てました。
>
> But because 薬 can be understood from context, it is left out in the same
> sentence.

When I looked up `期限切れ` in the dictionary*, it is listed as (Nouns which may
take the genitive case prt. 'no') I'm not sure what it means exactly so...

**(Question):** Is there something I need to understand differently because of
this dictionary categorisation? How are "Nouns which may take the genitive
case prt. 'no'" different from just nouns?

Would 日本語 in 日本語の先生 also be a "noun which may take the genitive case prt.
'no'"? Because so far many nouns are able to take の, and I dont understand the
separate category for it.

**EDIT:** I think this was what I meant to ask - did the `の` in question come
from elision or does it have something to do with 期限切れ being a no-adj ?

(Just looked it up in other dictionaries**, it's labelled as no-adj)

*taken from JMDict

**checked against WWWJDIC and japanese.nciku.com
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grammar particle-の formal-nouns
の performing back referral? の used adjectivally?
Your question is not really clear… Are you asking about "の" or "期限切れ"?

> の is used as a back referral

Actually, it's a simple elision..

誰のペンですか？ 私の(ペン)です。  
Whose pen is it? it is _mine_

何のタイアですか？ 車の(タイヤ)だ。  
Of what kind of vehicle is this tire? It's a car's one.

As far as "の for an adjective" goes, a famous example would be "色々" which can
go with の as well as な, to mean "various", but I'm not sure either how this
feat goes along the hint of your dictionary, if it's ever related.

> EDIT: I think this was what I meant to ask - did the の in question come from
> elision or does it have something to do with 期限切れ being a no-adj ?

**Edit** : Well, then both, probably :)  
Because 時間切れ works with a の, we could drop the word 薬 afterwards. If we were
considering a word without の, like "静か", we would have to use a palliative の
instead:  
その公園は去年静かなのだったので、寂しい。 (I'm sad because this park was a quiet one last year
(and isn't any more)).

**Edit2** : on の-adj.

There are some nouns that have a different function when used の. Consider the
nouns "普通" and "日本語". You say 普通の先生 and 日本語の先生, but while it is a "teacher of
Japanese" it is not a "teacher of normality". It's also not a "teacher from
the Japanese language" but it's an "average teacher". You are not working with
a genitive here, but you are really making an adjective that modifies the
following group.

As for why some adjectives work with の and others with な, I have no idea.
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Are no-adjs even a real thing in Japan? Any noun can modify another noun once
no is appended, and this modification may be rendered with an adjective in a
different language (eg 日本の文化 -> Japanese culture). It seems to me that 期限切れ is
no different in this regard. They could have translated it as "the condition
of being past the expiration date", and let the reader infer that 期限切れの means
"expired", but since the typical use is attributive anyway, they decided to
make things simpler for themselves and for foreign learners by translating
期限切れ[の] directly as "expired".
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