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.