  We present rotation period measurements for 41 field M-dwarfs, all of which
have masses inferred (from their parallaxes and 2MASS K-band magnitudes) to be
between the hydrogen burning limit and 0.35 Msol, and thus should remain
fully-convective throughout their lifetimes. We measure a wide range of
rotation periods, from 0.28 days to 154 days, with the latter commensurate with
the typical sensitivity limit of our observations. Using kinematics as a proxy
for age, we find that the majority of objects likely to be thick disk or halo
members (and hence, on average, older) rotate very slowly, with a median period
of 92 days, compared to 0.7 days for those likely to be thin disk members (on
average, younger), although there are still some rapid rotators in the thick
disk sample. When combined with literature measurements for M-dwarfs, these
results indicate an increase in spin-down times with decreasing stellar mass,
in agreement with previous work, and that the spin-down time becomes comparable
to the age of the thick disk sample below the fully-convective boundary. We
additionally infer that the spin-down must remove a substantial amount of
angular momentum once it begins in order to produce the slow rotators we
observe in the thick disk candidates, suggesting that fully-convective M-dwarfs
may still experience strong winds.
