Where [Hiro’s] body has bony extremities, the suit has sintered armorgel: feels like gritty jello, protects like a stack of telephone booths

So, sintered means this:

To form a material from fusible powder by holding the pressed powder at a temperature just below its melting point for a period of time; the particles are fused (sintered) together, but the mass, as a whole, does not melt.

Now, Technovelgy has an entry on sintered armorgel, and reckons that what Stephenson is describing is a dilatant fluid.

Thanks to the wonders of the intertubes, you can see what a dilatant fluid does right here:

Excess perspiration wafts through it

pretty much like the way Nike’s Sphere garment technology works:

Nike Dri-Fit
This high-performance microfiber polyester fabric actually pulls sweat away from the body and transports it to the fabric surface - where it evaporates and leaves the skin cool and dry. It’s all you need for hot days, and a critical base layer for cold days. Stay dry. Stay comfortable. No matter what.