I found that phrase applied a lot to women but then also to men (so that's probably not [only] related to being "in labour"). At first I thought it had to do with motherly/parenthood chores. But now I understand it's also applied to various unrelated activities.
And do we know the origin of the expression ?
From Princeton's WorldNet:
productive work performed voluntarily without material reward or compensation
Example usage:
Grandma's sweater for you was a labour of love, and yet you refused to wear it! She spent hours knitting it, and you threw it away like an old gym sock! You should be ashamed of yourself!
As you can see, "labour of love" is a selfless act.
It's a labor performed for the love of the work or for the target of the finished product. From TheFreeDictionary:
a labor undertaken on account of regard for some person, or through pleasure in the work itself, without expectation of reward.
In some ways it's similar to the concept of pro bono work (work done for free), but it normally is used to describe work someone really enjoys doing and is not compensated for.
Both answers given above regarding the meaning of labor of love are correct. And @muntoo's selfless act sums it up nicely in two words.
To answer the second part of your question, however, the first known use of the phrase is probably the one in the Bible, specifically the King James Version which translation dates back to 1611:
Remembering without ceasing your work of faith, and labour of love, and patience of hope in our Lord Jesus Christ, in the sight of God and our Father;
