IPA: /ˈtʃiːf.teɪn/
KK: /ˈtʃifˌteɪn/
A leader or head of a group, particularly one that belongs to a clan or tribe.
The chieftain called a meeting to discuss the tribe's future.
Chieftain is formed from "chief" (from Old French *chief*, meaning leader or head) and "-tain" (from Old French *tain*, a suffix denoting a person). The word refers to a person who is the leader or head of a group or tribe.
Think of a 'chief' as the leader, and the '-tain' just means a person. A chieftain is a person who leads.