Need help with uneven outline

[CHJ85]

Hi there. I know there's a way to even an uneven line that varies from i.e 1 to 3 pixels in thickness. Then you can make the line 2 pixels thick.
I can't for the life of me remember how I did that.
I remember I selected the uneven line(s), then did something to make them even. Not even sure if it was a plugin or a standard Gimp feature.
Any idea?
Thanks in advance