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.