when you durst do it, then you were a man

Note: Durst means dare.

To remember this quote use the following mnemonic.

When you dare do it, we'll do it!

Then you'll be a man.

"When you durst do it, then you were a man" act 1, scene 7, line 49.

The words here by Lady Macbeth shows her ambition by undermining Macbeths manhood. In the days of Shakespeare, the Jacobean era, it was considered a vital attribute. 

Lady Macbeth is a huge catalyst for Macbeths downfall and subsequent evil actions. Will Macbeth prove his masculinity to her?

Lady Macbeth is suggesting that only if Macbeth commits the murder of King Duncan could he then be considered a man.