This term was originally applied to both uranium and plutonium. U235 is the fissionable isotope, but makes up less that one percent of natural uranium. This has to be increased to 90% (by an extremely laborious process) to produce weapons grade uranium. Plutonium for weapons is generally produced in nuclear reactors (the far more commn U238 is easily transumuted to P239) but again it has to be refined to about 95% purity before it can be used to make a nuclear weapon.
Nowadays anthrax is often referred to as being of 'weapons grade', but the usual term applied to chemical and biological agents is that they have been weaponised. Weaponisation describes the process of making them more potent or virulent, and packaging them for efficient delivery to the target. Weapons grade is the more colorful phrase that looks good on a headline and sounds oh so cool when used by a teen to describe a fart, say, or a fiery chilli. Already a cliche.
Man that J-Lo sure has a weapons grade ass!
2. Bob likes to reload his own ammunition for his guns. He's experienced at what he's doing and so he only uses weapons grade materials.