The word arguendo is commonly translated to mean “for the sake of argument" or “ for argument’s sake." It is used in legal and academic settings to indicate when the speaker is about to make a point and is asking the listener to take an assumption for granted. For example, were a speaker to say:
“Arguendo, let’s say that Michael did indeed enter the dry cleaning store on the day in question. Does this make him guilty of robbery?"
The speaker in this case is asking the listener to entertain an unsubstantiated statement or assumption. Notice that the speaker’s question is dependent on the previous arguendo statement.
As with many Latin phrases used in English, the literal translation of arguendo has been lost in favor of a more convenient metaphoric meaning. The modern translation “for the sake of argument" is not technically correct.
The word arguendo is derived from the Latin word arguo (aguere, argui, argutum), a second conjugation verb meaning “to make clear," “to declare," or “to prove." The –nd– portion of the word gives away that the verb is in its verbal noun or gerund form. Recall that English verbs can be turned into gerunds by adding –ing to the verb’s stem. For example:
like (verb), liking (gerund)
walk (verb), walking (gerund)
Putting a verb in its gerund form essentially turns a verb into a noun which may then, for example, function as the subject of a sentence.
Given the –o ending, arguendo may be either dative or ablative. In this particular case, the word is in the ablative and is a good example of the ablative of manner. This ablative construction is used to indicate in what manner something is done. Coupled with its gerund form, arguendo may be appropriately translated as “by arguing." Were we interested in a literal translation into Latin for the phrase “for the purpose of argument," an appropriate translation would be “gratia argumenti" which is not unlike ars gratia artis, “art for the purpose [sake] of art."