Dour may imply grumpiness, not necessarily rudeness or unkindness. His dour expression seemed off-putting, but he greeted everyone politely.
Pro Tip 2/3
Subdued Emotion
Dour refers to a lack of visible emotion rather than just sadness. She remained dour during the celebration, barely cracking a smile.
Pro Tip 3/3
Masked Kindness
Someone may appear dour but can exhibit kindness or warmth unexpectedly. The teacher's dour face intimidated students, yet he was always ready to help.