The phrase Person X - to mean a generic person - has been around for about 50 years:

Ngrams, person X is lower than Mrs X which is lower than Mr X

What term was used before it to denote a generic person?
Was it all John Smith et al?

From where does the phrase Person/Mr/Mrs X originate and does it have a link to the use of 'X' in algebra?

  • I'm guessing it comes from algebra. Likely going back to Greek mathematicians. – Hot Licks Oct 14 '16 at 12:28
  • Could whoever popped by simply to down-vote the question explain why? – BladorthinTheGrey Nov 13 '16 at 19:34

According to etymonline the use of X to refer to an unknown person is from the end of the 18th century,

  • Used allusively for "unknown person" from 1797.

I think that the usage was later extended to refer to a "generic person".Ngram shows usages from the 19th century of "person X/x".

From: Equity, Also, The Forms of Actions at Common Law- 1913:

  • If therefore — this is the simplest application of the rule — a testator has affected to dispose of property which is not his own and has also given some benefit to the person, X, to whom that property belongs, that person, X, if he accepts the benefit thus destined for him by the testator, must make good the testator's attempted ...

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