  We present the projected rotational velocity and molecular abundances for HD
33632 Ab obtained via Keck Planet Imager and Characterizer high-resolution
spectroscopy. HD 33632 Ab is a nearby benchmark brown dwarf companion at a
separation of $\sim$20 au that straddles the L/T transition. Using a
forward-modeling framework with on-axis host star spectra, self-consistent
substellar atmospheric and retrieval models for HD 33632 Ab, we derive a
projected rotational velocity of 53 $\pm$ 3 km/s and carbon/water mass
fractions of log CO = $-$2.3 $\pm$ 0.3 and log H$_2$O = $-$2.7 $\pm$ 0.2. The
inferred carbon-to-oxygen ratio (C/O = 0.58 $\pm$ 0.14), molecular abundances,
and metallicity ([C/H] = 0.0 $\pm$ 0.2 dex) of HD 33632 Ab are consistent with
its host star. Although detectable methane opacities are expected in L/T
transition objects, we did not recover methane in our KPIC spectra, partly due
to the high $v\sin{i}$ and to disequilibrium chemistry at the pressures we are
sensitive to. We parameterize the spin as the ratio of rotation over break-up
velocity, and compare HD 33632 Ab to a compilation of >200 very low-mass
objects (M$\lesssim$0.1 M$_{\odot}$) that have spin measurements in the
literature. There appears to be no clear trend for the isolated field low-mass
objects versus mass, but a tentative trend is identified for low-mass
companions and directly imaged exoplanets, similar to previous findings. A
larger sample of close-in gas giant exoplanets and brown dwarfs will critically
examine our understanding of their formation and evolution through rotation and
chemical abundance measurements.
