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

tenured graduate student
 n.

    One who has been in graduate school for 10 years (the usual maximum is 5 or
    6): a ?ten-yeared? student (get it?). Actually, this term may be used of
    any grad student beginning in his seventh year. Students don't really get
    tenure, of course, the way professors do, but a tenth-year graduate student
    has probably been around the university longer than any untenured
    professor.


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

tenured graduate student

    One who has been in graduate school for 10 years (the
   usual maximum is 5 or 6): a "ten-yeared" student (get it?).
   Actually, this term may be used of any grad student beginning
   in his seventh year.  Students don't really get tenure, of
   course, the way professors do, but a tenth-year graduate
   student has probably been around the university longer than
   any untenured professor.

   [Jargon File]

   (1996-09-27)