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.