The etymology of maror מרור (in the Tanach, it never appears in the singular but rather as the plural merorim מרורים) is simple - it means "bitter herbs", from mar מר - "bitter".
What English word also derives from the same Hebrew root? The spice myrrh gets its name from the Hebrew word mor מור. This spice was burned at the altar in the Temple, and appears numerous times in Shir HaShirim. Rashi (Bereshit 22:2), following Onkelos, explains the etymology of Har HaMoriah as coming from the spice mor. While other explanations are given, Shir HaShirim (4:6) does mention Har HaMor.
This of course leads to the question: why would such a central spice be named after something so bitter? This site seems to give the answer:
Myrrh is a gum resin produced by trees and shrubs of the family Burseracea, most notably Commiphora myrrha, Commiphora abysinica, and Commiphora schimperi. The resin is obtained from Arabia and adjacent Africa, and is taken from the small, prickly gray-barked trees. Pearls of myrrh are brown, red or yellow, with an oily texture, becoming hard and brittle with age. It has a pleasing fragrance, very much like balsam, and a lasting, bitter, aromatic taste, hence the name mor, which signifies bitterness.
I'm sure there's a nice drasha that can be made about how mor is sweet to smell but bitter to taste. If someone knows of one, please let me know; otherwise feel free to write your own...