Interesting definitions on Quick Definitions for onelook.com:

"adjective: (of a horse) having horseshoes with projecting nails to prevent slipping
adjective: unjustly domineering (Example: "A manager who rode roughshod over all opposition")
adjective: (of persons or their actions) able or disposed to inflict pain or suffering"

I've never used this word although I've read it, and I found it to be an interesting synonym for 'vicious' in one thesaurus. Interesting to see the equine connection.