Playing through PoE I noticed that after you defeat Maerwald, Aloth says some funky stuff and then says "Damnit Virago, not now!  Later on we learn that his awekened soul is called Iselmyr.  I did a search and could find nothing relevant to this, so my question is who/what is "Virago"?

According to Merriam-Webster, a virago is:

 

1 : a loud overbearing woman : termagant

2 : a woman of great stature, strength, and courage

 

So it's not so much a name but an insult/epithet levelled at Iselmyr without giving out her name to the player.

His actual words are "Damned virago, not now!" So with the definition above, hopefully they make sense now.