What are its origins and what does it really mean?
The concept behind the phrase is that some areas of life are so important and overwhelming that you cannot blame someone for acting in their own best interest. For war, this implies that spies, torture, lying, backstabbing, making deals with enemies, selling out allies, bombing civilians, wounding instead of killing, and so on are "fair game" in the sense that by taking these options off of the table you are only hurting yourself: Your opponent has no reason to comply to your moral standards. (This entire concept is mostly void with regards to the current political atmosphere of Earth. Countries have actually declared certain things taboo with regards to war — with mixed success.)
The point of adding love to the list is likely to compare it to war. There are two main subtexts here. The first and most relevant is the idea that you can wreak all the havoc you want during the pursuit of true love. This includes sabotaging the third side in a love triangle or using deceit and trickery to woo the object of your affection (including hiding past lovers from them).
The second is the viewpoint that ongoing love between two people is akin to a battle that results in a dominant winner. The stereotypical gender wars are similar to this. A man and woman are in love, but a certain unease comes with the territory, and pulling one over on your spouse is fair game because, in the end, all is fair in love and war. For what it is worth, this last point is probably more of a causality (i.e., an unintended, natural consequence) of the individual words in the phrase than any original intended meaning. Most people probably don't immediately think of this type of behavior as matching up with the phrase until after they need an excuse for their actions.
I'll paste the definition given by the OALD because it's clearer than how I may say it: "In some situations any type of behaviour is acceptable to get what you want", i.e. you can be as deceitful, dishonest and false as you want in war and love.
I don't think anyone has addressed (more than cursorily, anyway) the part of the poster's question that asks, "What are its origins?"—so I'll focus on answering that. J. A. Simpson, The Concise Oxford Dictionary of Proverbs (1982) offers this lineage for the proverb:
But between Shelton in 1620 and James in 1845, an interesting alternative commentary on love and war was developing in North America. From Bartlett Whiting, Early American Proverbs and Proverbial Phrases (1977):
Martin Manser, The Facts on File Dictionary of Proverbs (2002) cites the Smedley quotation from 1850 as the first instance of the proverb in its current standard form, but then notes:
Linda and Roger Flavell, Dictionary of Proverbs and Their Origins (1993) offers this longer view of "all's fair in love and war":
Rosalind Fergusson, The Facts on File Dictionary of Proverbs (1983)—same title and publisher as Manser's book above, but almost twenty years older and completely different in content—identifies several allied proverbs, though it doesn't identify where these other sayings come from:
protected by RegDwigнt♦ Feb 6 '13 at 10:27
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