嘘なわけ in this sentence?

Question

それは嘘なわけだが、

I don't understand. Does it say "it's a lie" or "it may be a lie"? I think I've read from a previous thread that な here is the equivalent of だ, but I'm stuck at わけ.

Answer 1

My dictionary says this type of わけ is used to confirm a (known) fact before diving into a main discussion. Context is missing, but I feel the speaker is trying to imply it's an obvious lie. So the nuance is something like "Of course it's a lie, but ..." or "As you know, it's a lie, but ..." Naturally, it may sound scornful if said referring to someone else's statement.

• 知っての通り、我々の会社は赤字を出している訳ですけれども、…
• ハハハ、まあ今言ったことは冗談な訳ですが、今から言うことは大事です。…
• さて、今日は私の誕生日な訳だけど、プレゼントはどこ？ まさか忘れたの？

な is an attributive form of copula だ. It typically appears before explanatory-の, もの, ので or のに. But in casual speech it's sometimes used before こと, わけ, 理由, 場合, etc.

• 彼はもう大学生なのだ。
• 彼はもう大学生なのに、マナーを知らない。
• 彼が大学生なものか！
• 彼が大学生なことを知らなかった。 (大学生であること is better in formal settings)
• 彼が大学生な訳がない。 (大学生である訳/大学生の訳 is better in formal settings)
• 大学生な人 (nonstandard and highly informal, but occasionally heard instead of 大学生の人/大学生である人)

Answer 2

My dictionary says that this わけだ is the soft way of saying of だ that means "conclusion".

It means ""it's a lie". It is the equivalent of それは嘘だが, but the author would think it is a bit strong.

Answer 3

It means "It's a lie."

I think I've read from a previous thread that な here is the equivalent of だ,

This isn't quite correct. You cannot substitute な for だ to end a sentence or following a -na adjective or noun preceding the と particle. In this case, わけ requires that its qualifying clause have an adjectivized suffix if it doesn't end in a verb (i.e. you need one if it ends in any kind of adjective or noun), so nouns end up following the pattern of -na adjectives and have a な suffix added.

So 嘘なわけ is the same in terms of grammar as saying 面白いわけ or 綺麗なわけ.