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."
See also overflow bit.