bonnet rouge

from The Century Dictionary.

• noun The cap of liberty of the French revolutionists of 1793. See liberty-cap.
• noun Hence A wearer of such a cap; a sans-culotte.
• noun A red republican; an anarchist or communist.

from the GNU version of the Collaborative International Dictionary of English.

• The red cap adopted by the extremists in the French Revolution, which became a sign of patriotism at that epoch; hence, a revolutionist; a Red Republican.