diagonal: [16] Diagonal is commonly used simply as a synonym for oblique, but in strict mathematical terms it denotes a line joining two non-adjacent angles of a polygon. This reveals far more clearly its origins. It comes from diagōnālis, a Latin adjective derived from Greek diagónios. This was a compound formed from the prefix dia- ‘across’ and gōníā ‘angle’ (as in English polygon), meaning ‘from angle to angle’. Gōníā is related ultimately to English knee and genuine. => genuine, knee, polygon[diagonal etymology, diagonal origin, 英语词源]