early 14c., an ecclesiastical title, etymologically "head of a group of ten," from Old French deien (12c., Modern French doyen), from Late Latin decanus "head of a group of 10 monks in a monastery," from earlier secular meaning "commander of 10 soldiers" (which was extended to civil administrators in the late empire), from Greek dekanos, from deka "ten" (from PIE root *dekm- "ten"). It replaced Old English teoðingealdor.
Sense of "president of a faculty or department in a university" is by 1520s (in Anglo-Latin from late 13c.). Extended meaning "oldest member in length of service in any constituted body" is from mid-15c. Related: Deanery.