What is the meaning/usage of いざ?

Question

Will someone please explain what this word いざ means? I've found several definitions, but I'm very unclear on what it really means and its usage. I've heard it mostly in the following ways:

• ～（は・なら）いざ知らず　→　"I don't know about ～, but ..."　→　This pattern makes sense to me, but it's about the only one.
• いざ尋常【じんじょう】に　→　I've heard this in the video game series Samurai Shodown (さむらいスピリツ). The announcer says it before the fight, and it means something like "Fair and square". EDIT: The full pre-round announcement in the game is いざ尋常に...（一・二・三）本目...勝負【しょうぶ】！

EDIT: I also saw this on a Japanese friend's Facebook picture. The caption said いざ新天地へ and the picture is of their child inside a cardboard moving box. This seems to correspond to "After all this time, finally (we're moving!)" that @sawa posted below.

Even though I understand the pattern ～（は・なら）いざ知らず, it would seem to still make sense even without the いざ in it. It seems that いざ is some type of emphasis word or interjection.

How do you use it? When can you use it? What types of words can you use it with?

Answer 1

As Chocolate wrote in a comment, there were two different words いざ and いさ in classical Japanese.

いざ is an interjection used when the speaker encourages someone to do something together or the speaker starts something important enthusiastically. In modern Japanese, さあ is much more common for this purpose. I do not think that there is a perfectly corresponding word in English. Example: いざ行かん = さあ行こう = Let’s go!

The いざ in いざ尋常に is this word, but it probably requires more explanation. いざ尋常に is short for いざ尋常に勝負せよ. 尋常 has several meanings, and in this example it means “without unseemly fuss.” So いざ尋常に means something like “Fight a duel against me without trying to avoid it.”

いさ is an interjection used when the speaker expresses unsureness about what was being asked. Interestingly, it is also さあ in modern Japanese. It is also used as an adverb in the form いさ知らず, in which case it just emphasizes the 知らず part.

In later time, いざ and いさ were conflated and いざ has been used in both meanings.