冗 談 じゃない — Tae Kim's Japanese Grammar flashcard

[This (e.g. what I just said)] is not a joke!

"janai" is the negative form of "da". Tae Kim explains this as "negative state-of-being". So "joudan janai" literally declares "joudan" to be not a joke. E.g. [something that was said before] is not a joke! This is serious!

Reminder 1: In []-brackets I provide additional information that's clear from context but not explicitly stated in words. In Japanese, lots of things that are clear from context are omitted.Reminder 2: Japanese also has words like "this" or "that", but they are usually omitted - especially when the context these words refer to is known.