So, you've got a character that you want to show is really dangerous. You could give them the normal two arms or legs, but that's just not enough. Maybe you could replace one of those arms with a really big gun, or a sword, or have it transform into something else.
If that doesn't work, you can always give a character extra arms... because two arms good, four arms better, right?
For obvious reasons, normal humans don't get this treatment, because it would look pretty awkward having to do bodily cleaning... and having four arms to go around would be a tailor's nightmare. If you're a mutant or an evil genius, you don't have to worry about this, because Freaky Is Cool.
Multi-Armed and Dangerous characters come in two flavors:
The extra arms are a regular part of the body.
They're made from prosthetics/synthetic materials that are usually attached to the person.
They often have Super Strength. Their ability to fight with extra arms is a fantastic form of Heroic Ambidexterity. Combat Tentacles are a subtrope. Occasionally you'll see such characters fighting with a weapon for each of their hands. Can sometimes be used as Spider Limbs.
This isn't what they mean, by the way, on the news when they say "The local bank was robbed by four armed men", but "To be forewarned is to have four arms."
As of the 21st century, there is (very expensive) technology that allows a person to control an artificial limb with the same mental process as an organic limb and can be used to control extraneous limbs with only a minimal period of adjustment. Truth in Television is only a matter of time...
Sister Trope to Multiarmed Multitasking, and they're the children of Vertebrate with Extra Limbs.
For other extra body parts, see Extra Digits, Extra Eyes, Multiboobage, Triple Nipple, and Multiple Head Case.
