맹

Promise 

맹세(하다) = vow, oath 
동맹 = alliance
연맹 = union

Fierce

맹수 = ferocious beast 
맹독 = deadly poison
맹견 = ferocious dog

Blind

맹인 = blind person
색맹 = color-blind person
문맹 = illiterate person
맹장 = appendix, blind gut
맹신(하다) = believe blindly
맹인안내견 = guide dog