barn n.
[uncommon; prob. from the nuclear military] An unexpectedly
large quantity of something: a unit of measurement. "Why is
/var/adm taking up so much space?" "The logs have grown to
several barns." The source of this is clear: when physicists were
first studying nuclear interactions, the probability was thought
to be proportional to the cross-sectional area of the nucleus
(this probability is still called the cross-section). Upon
experimenting, they discovered the interactions were far more
probable than expected; the nuclei were `as big as a barn'. The
units for cross-sections were christened Barns, (10^-24 cm^2) and
the book containing cross-sections has a picture of a barn on the
cover.