Why do khukuris have that notch? What purpose does a cho have? Does it weaken the knife?

[Kailash Blades]

This was my belief when I first started collecting khukuries. While I hear what you are saying, I don't know if it is correct to assume that the spine would be covered in fluid during normal use. Its the front of the blade that makes contact as the knife swings through. That would likely also be the case if I were using it as a skinner or processing sticks or cuts of beef. Also , whatever fluid may contaminate the spine would tend to flow towards the edge as the knife is held upright during use, and then either drop off the blade or flow along the edge towards the handle. So it would make sense to have a feature to interrupt the flow in between the edge and handle.

n2s

It definitely makes sense as a practical feature. If it interrupts to flow it could lead to a better grip during butchering or fighting and would be very handy. While there is a wide variety of different cho shapes they do usually have some kind of gap, tip or protrusion at the bottom of the edge that could fulfill this purpose. In my experience with a few different cho shapes they don't really help that much with this aspect though. If you could try it with your own blades and report back it would be great to get a wider sample size.
Take care,
Andrew and the team at Kailash