The territory or domain ruled by chieftains; a system or collection of chieftains; the rank or office of a chieftain.
From 'chieftain' (Old French chevetain) plus '-ry' suffix, which denotes a domain, practice, or collection. The '-ry' suffix has been creating abstract nouns in English since Middle English times.
Languages use suffixes like '-ry' to turn concrete nouns into abstract concepts—'flower' becomes 'flowery' (characterized by flowers), and 'chieftain' becomes 'chieftainry' (the system of chieftains), letting us discuss entire organized structures with just one word.
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