The Collaborative International Dictionary of English v.0.48:

Chieftaincy \Chief"tain*cy\, Chieftainship \Chief"tain*ship\, n.
   The rank, dignity, or office of a chieftain.
   [1913 Webster]

WordNet (r) 3.0 (2006):

chieftaincy
    n 1: the position of chieftain [syn: chieftaincy,
         chieftainship]

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "chieftaincy":
   aedileship, ally, archbishopric, archduchy, archdukedom,
   archiepiscopacy, archiepiscopate, aristocracy, bishopric,
   body politic, buffer state, captive nation, chairmanship,
   chancellery, chancellorate, chancellorship, chiefery, chiefry,
   chieftainry, chieftainship, city-state, colony, commonweal,
   commonwealth, consulate, consulship, country, county, deanery,
   dictatorship, dictature, directorship, domain, dominion, duchy,
   dukedom, earldom, emirate, empery, empire, episcopacy, free city,
   governorship, grand duchy, headship, hegemony, hierarchy, kingdom,
   land, leadership, lordship, magistracy, magistrateship,
   magistrature, mandant, mandate, mandated territory, mandatee,
   mandatory, masterdom, mastership, mastery, mayoralty, mayorship,
   metropolitanate, metropolitanship, nation, nationality, nobility,
   papacy, pashadom, pashalic, patriarchate, patriarchy, polis,
   polity, pontificality, pontificate, popedom, popehood, popeship,
   possession, power, prefectship, prefecture, premiership,
   presidency, presidentship, prime-ministership, prime-ministry,
   princedom, princeship, principality, principate, proconsulate,
   proconsulship, protectorate, protectorship, province, provostry,
   provostship, puppet government, puppet regime, realm, rectorate,
   rectorship, regency, regentship, republic, ruling class, satellite,
   seigniory, seneschalship, seneschalsy, seneschalty, settlement,
   sheikhdom, sheriffalty, sheriffcy, sheriffdom, shrievalty,
   sovereign nation, state, sultanate, superpower, supervisorship,
   suzerainship, suzerainty, territory, toparchia, toparchy,
   tribunate, vizierate, viziership