To declare formal revenge or vendetta on your enemy or archnemesis, and in the process, letting them know the revenge is on. Sometimes this is done to protect outsiders from the conflict. To declare kanly means that you will start your revenge, expect your enemy to respond in kind, and not quit until the revenge is complete through eternal family and honor shaming, and/or death. The type of vengence needed for kanly is usually a multi-generational, traditional duty bound blood feud of the worst kind. Kanly should not be taken lightly.

"KANLY: formal feud, vendetta under the rules of the Great Convention carried on according to the strictest Limitations. (See Judge of the Change.) Originally the rules were designed to protect innocent bystanders." - Dune Glossary

When you eat dirt for the first time and leave your family to build a shack in the woods

To publicly, display, polite disdain for an opponent, or rival.