It is so because of different origins of the word "discourse" ( a logical formal argument or speech, from the French, discours ) and the Latin dis+cursive. Dis (the prefix) means apart; as in dis honest, dis cord; the cursive comes from the verb to run (as in cursive writing). So it literally means "running apart" or "running every which way" Some of my old professors did tend to ramble, and to delve into long discourse.
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