외삼촌이 · 외삼촌을
maternal uncle (mother's brother)
Example
외삼촌은 어머니의 남동생이에요.
My maternal uncle is my mother's younger brother.
Word parts
외 outside+삼 three+촌 degree
외 marks the mother's side; 삼촌 means 'three degrees' of kinship.
Good to know
외할머니, 외할아버지, 외삼촌.