I've known what caltrops were since I was a kid: those four-pointed metal things, scattered in roadways to damage tires or hooves or feet.
But as with so many words, somehow it didn't occur to me 'til recently to wonder about this word's origins. It turns out that the word "caltrop" (also spelled "calthrop") also refers to any of a variety of plants (the star thistle, the puncture vine, and others) that bear spikes. (And/or look like they bear spikes.)
I'm a little uncertain how the etymology fits together (because Wikipedia seems to suggest that at least one of the plants was named after the weapon, rather than the other way 'round--but then, I don't really trust Wikipedia to be 100% accurate about such things), but MW3 indicates that the word derives from Medieval Latin calcatrippa, which probably derives eventually from roots meaning "heel" and "trap."
Meanwhile, a "caltrap" or "galtrap" is a heraldic representation of the weapon. Like the device at the top of this Heraldry of some Yorkshire Families page. Would that be azure three caltraps or? I loved heraldry when I was a kid (all those cool weird words!), but I'm way rusty at this point.