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

lost in the underflow
 adj.

    Too small to be worth considering; more specifically, small beyond the
    limits of accuracy or measurement. This is a reference to floating
    underflow, a condition that can occur when a floating-point arithmetic
    processor tries to handle quantities smaller than its limit of magnitude.
    It is also a pun on ?undertow? (a kind of fast, cold current that sometimes
    runs just offshore and can be dangerous to swimmers). ?Well, sure, photon
    pressure from the stadium lights alters the path of a thrown baseball, but
    that effect gets lost in the underflow.? Compare epsilon, epsilon
    squared; see also overflow bit.


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

lost in the underflow

    Too small to be worth considering; more specifically,
   small beyond the limits of accuracy or measurement.  This is a
   reference to "floating point underflow".

   The Hacker's Jargon File claimed that it is also a pun on
   "undertow" (a kind of fast, cold current that sometimes runs
   just offshore and can be dangerous to swimmers).

   "Well, sure, photon pressure from the stadium lights alters
   the path of a thrown baseball, but that effect gets lost in
   the underflow".

   Compare epsilon, epsilon squared; see also overflow bit.

   (1997-09-05)