Well according to Wikipedia, a shieldmaiden is:
a virgin who had chosen to fight as a warrior in Scandinavian folklore and mythology and they are often mentioned in sagas such as Hervarar saga and in Gesta Danorum. Shieldmaidens also appear in stories of other Germanic nations: Goths, Cimbri and Marcomanni. The Valkyries were based on the shieldmaidens, and they were J.R.R. Tolkien's inspiration for Éowyn.
So basically, a shieldmaiden could be considered as the female counterpart of "a knight in shining armor". She was a soldier; a woman or girl who chose to fight for her country, beliefs, cause, or for whatever other reasons she might have had. St. Joan of Arc could be considered a sheildmaiden, then, I suppose.
But what does it mean to be a shieldmaiden toady? Obviously many of us aren't part of Scandinavian folklore or mythology, and even fewer of us are soldiers in the army, or likely to become one. How can one be a shieldmaiden then?
I found an atricle about Shieldmaidens on the website chivalrytoday.com. While I don't agree with most of the article (it has a slightly feminist-ic stance), there were a couple of good paragraphs in there. I take the liberty to quote them now.
You may say, "Yes, this is all fine and good, but is that it?" The answer, in my opinion, is no. We as Christians can become shieldmaidens by becoming warriors of Heaven. By putting on our spiritual armor and fighting battles for the greater glory of God. Anyone can fight for the truth and for the faith. By taking up our spiritual shields, we can become Shieldmaidens in the purest and holiest meaning of the word.
A shieldmaiden is firstly a woman. As a woman she is the keeper of the hearth flame that provides a safe nurturing environment for her family. She is the teacher of her children and lady for her lord. The shieldmaiden has the courage to do the unglamorous daily tasks and to do them graciously. She is an example to her family and the keeper of the peace. She is generous to others while also being generous to herself. She keeps her word no matter how hard to fulfill and she stands by her beliefs. She selflessly serves......As a warrior a shieldmaiden must have the strength to make her way in a man’s world. She must be willing to get her hands dirty and make sacrifices. She may have to bide her time in an unsavory position until the time to fight is at hand. When that time comes she must have the courage to stand without blame. She keeps her word and is willing to show mercy. She is willing to help others. She stands by her convictions and commitments.
We are soldiers of Christ and shieldmaidens of God.