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.

adjective: … A manager who rode roughshod

Interesting. I would have thought it was an adverb.

interesting synonym for 'vicious' in one thesaurus. Interesting to see the equine connection.

I think it's the nails in the shoes, wordwind.

If you walk all over someone with nails in your shoes, that's vicious, real "vicious".

The Viet Cong used "naily boards" against their enemies. They would rather maim than kill because it put two additional soldiers out of action attending to the wounded.

Vietnam era U.S. Army Footwear

Standard black leather combat boots proved hot and susceptible to rot. The military introduced jungle boots, which included cooler nylon-mesh uppers, and drain holes that allowed water to escape. The boots also featured reinforced soles to protect against the sharpened bamboo stakes, or "punji spikes," used as booby traps by enemy soldiers.

http://www.pbs.org/wgbh/amex/vietnam/trenches/weapons.html

Faldage, great eye! I should have caught that when reading the onelook definitions! You are absolutely right about that sentence in which roughshod is an adverb and not an adjective. I'll tip the cup to you tonight.

I thought Sparteye had the great eye around here, Wordwind. BTW I'll tip one to Faldage too if you're buyin'. :) [Can themilum be far behind?]

Just checked it out for myself, Wordwind:

Function: adverb
: in a roughshod manner <rode roughshod over the opposition>

I never would have caught that adverb either. So I might even pay for my own cup. [Can't speak for themilum, however.] P.S. "Roughshod" in "roughshod manner" is an "adjective", isn't it? If so, M-W gave us one of each.