Sorry, wasn't quite finished on the post, had to leave a few minutes. It is found on the Lakota pipe axes and hawks. Must be for the warrior. The heart is an ancient symbol going back to ancient times. It represented the womb, fertility. Buffalo horns make it associated with North, warrior, Earth.
Heart as a symbol
http://en.wikipedia.org/wiki/Heart_%28symbol%29
You know, I can't find anywhere mentioned as a fertility symbol. This is something Wicca schools used to teach.
In fact from Wikipedia (above)
However, more realistic depictions of human hearts tend to have macabre connotations of death and violence, quite unlike the concepts associated with the poetic and symbolic heart.
I tried Wilkipedia for Lakota Weeping Heart and it came up blank. So, I tried Google, my post here came up in the results. The other places it showed up was Native weapons, war.
I've never really seen any info on it. I know what they use it on, smoking axes. Also just plain Tomahawks that were used as a weapon.
Native Tomahawk, shows the peace pipe one.
http://en.wikipedia.org/wiki/Tomahawk_%28axe%29
I think Weeping Heart is specifically a Lakota trademark, so everyone will know it is Lakota. I've only seen it on blades. Someone who is Lakota could probably explain the origin of the symbol.
I've used my Tomahawk pipe with the Weeping Heart mostly in healing ceremonies. To me it is a "healing heart" and is good for the heart.