If someone calls you a gutless coward, he's telling you that you lack both courage and viscera. Your viscera are your innards or your guts. In popular usage the term refers to the intestines, but technically it includes all soft internal organs.
Viscera comes from the Latin viscus, meaning "an inner part of the body." You won't hear it much nowadays unless you're studying anatomy or describing particularly gory special effects. (Parents object to ultra-violent videogames where bodies explode, spewing viscera.) Don't confuse it with the related visceral, which does crop up often today and means "instinctive." "She had a visceral reaction to seeing her boyfriend and his hunting pals covered in deer blood and viscera, and dumped him the next day."