Part of Speech: Verb

Definition: To feel or express grief or sorrow; to feel or express grief or sorrow.

Source: https://www.merriam-webster.com/dictionary/mourn

Found in: “My Boy Willie” by Anonymous line 36 – “Ye must mourn in black for my sailor boy”

The word mourn means to feel upset because of a  close ones passing. This can help me understand the poem better because it explains why everyone has to wear black, and what they are doing for the speakers dear sailor boy.