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

copious free time
 n.

    [Apple; orig. fr. the intro to Tom Lehrer's song It Makes A Fellow Proud To
    Be A Soldier]

    1. [used ironically to indicate the speaker's lack of the quantity in
    question] A mythical schedule slot for accomplishing tasks held to be
    unlikely or impossible. Sometimes used to indicate that the speaker is
    interested in accomplishing the task, but believes that the opportunity
    will not arise. ?I'll implement the automatic layout stuff in my copious
    free time.?

    2. [Archly] Time reserved for bogus or otherwise idiotic tasks, such as
    implementation of chrome, or the stroking of suits. ?I'll get back to
    him on that feature in my copious free time.?


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

copious free time

    (Apple; originally from the introduction to Tom
   Lehrer's song "It Makes A Fellow Proud To Be A Soldier") Used
   ironically to indicate the speaker's lack of the quantity in
   question; a mythical schedule slot for accomplishing tasks
   held to be unlikely or impossible.  Sometimes used to indicate
   that the speaker is interested in accomplishing the task, but
   believes that the opportunity will not arise.  "I'll implement
   the automatic layout stuff in my copious free time."

   The phrase is also used for time reserved for bogus or
   otherwise idiotic tasks, such as implementation of bad
   chrome, or the stroking of suits.  "I'll get back to him
   on that feature in my copious free time."

   [Jargon File]

   (1994-11-30)