The Jargon File (version 4.4.7, 29 Dec 2003):

for values of


    [MIT] A common rhetorical maneuver at MIT is to use any of the canonical 
    random numbers as placeholders for variables. ?The max function takes 42
    arguments, for arbitrary values of 42.:? ?There are 69 ways to leave your
    lover, for 69 = 50.? This is especially likely when the speaker has uttered
    a random number and realizes that it was not recognized as such, but even
    ?non-random? numbers are occasionally used in this fashion. A related joke
    is that ? equals 3 ? for small values of ? and large values of 3.

    Historical note: at MIT this usage has traditionally been traced to the
    programming language MAD (Michigan Algorithm Decoder), an Algol-58-like
    language that was the most common choice among mainstream (non-hacker)
    users at MIT in the mid-60s. It inherited from Algol-58 a control structure
    FOR VALUES OF X = 3, 7, 99 DO ... that would repeat the indicated
    instructions for each value in the list (unlike the usual FOR that only
    works for arithmetic sequences of values). MAD is long extinct, but similar
    for-constructs still flourish (e.g., in Unix's shell languages).


The Free On-line Dictionary of Computing (30 December 2018):

for values of

    A common rhetorical maneuver at MIT is to use any
   of the canonical random numbers as placeholders for
   variables.  "The max function takes 42 arguments, for
   arbitrary values of 42".  "There are 69 ways to leave your
   lover, for 69 = 50".  This is especially likely when the
   speaker has uttered a random number and realises that it was
   not recognised as such, but even "non-random" numbers are
   occasionally used in this fashion.  A related joke is that pi
   equals 3 - for small values of pi and large values of 3.

   This usage probably derives from the programming language MAD
   (Michigan Algorithm Decoder), an ALGOL-like language that
   was the most common choice among mainstream (non-hacker) users
   at MIT in the mid-1960s.  It had a control structure FOR
   VALUES OF X = 3, 7, 99 DO ... that would repeat the indicated
   instructions for each value in the list (unlike the usual FOR
   that generates an arithmetic sequence of values).  MAD is
   long extinct, but similar for-constructs still flourish
   (e.g. in Unix's shell languages).

   [Jargon File]

   (1994-12-16)