1540s, from Medieval Latin tendonem (nominative tendo), altered (by influence of Latin tendere "to stretch") from Late Latin tenon, from Greek tenon (genitive tenontos) "tendon, sinew," from PIE *ten-on- "something stretched," from root *ten- "to stretch."
word-forming element in medicine denoting "diseases characterized by inflammation" (of the specified part), Modern Latin, from Greek -itis, feminine of adjectival suffix -ites "pertaining to." Feminine because it was used with an implied nosos "disease," a feminine noun; especially in arthritis (nosos) "(disease) of the joints." Arthritis (16c.) was one of the earliest appearances of the suffix in English and from it the suffix was abstracted in other uses.