  We demonstrate that the previously identified L/T transition brown dwarf SDSS
J213154.43-011939.3 (SDSS 2131-0119) is a widely separated (82.3'', $\sim$3830
au) common proper motion companion to the low-mass star NLTT 51469, which we
reveal to be a close binary itself, separated by 0.64''$\pm$0.01'' ($\sim$30
au). We find the proper motion of SDSS 2131-0119 of
$\mu_{\alpha}\cos\delta=-100\pm20$, $\mu_{\delta}=-230\pm20$ mas/yr consistent
with the proper motion of the primary provided by Gaia DR2:
$\mu_{\alpha}\cos\delta=-95.49\pm0.96$, $\mu_{\delta}=-239.38\pm0.96$ mas/yr.
Based on optical and near-infrared spectroscopy we classify NLTT 51469A as a
M3$\pm$1 dwarf, estimate photometrically the spectral type of its close
companion NLTT 51469B at $\sim$M6 and confirm the spectral type of the brown
dwarf to be L9$\pm$1. Using radial velocity, proper motion and parallax we
derived the $UVW$ space velocities of NLTT 51469A, showing that the system does
not belong to any known young stellar moving group. The high $V, W$ velocities,
lack of 670.8 nm Li I absorption line, and absence of H$\alpha$ emission,
detected X-rays or UV excess, indicate that the system is likely a member of
the thin disk population and is older than 1 Gyr. For the parallactic distance
of $46.6\pm1.6$ pc from Gaia DR2 we determined luminosities of
$-1.50^{+0.02}_{-0.04}$ and $-4.4\pm0.1$ dex of the M3 and L9, respectively.
Considering the spectrophotometric estimation which yields a slightly lower
distance of $34^{+10}_{-13}$ pc the obtained luminosities are
$-1.78^{+0.02}_{-0.04}$ and $-4.7^{+0.3}_{-0.5}$ dex. We also estimated their
effective temperatures and masses, and obtained 3410$^{+140}_{-210}$ K and
$0.42\pm0.02 M_{\odot}$ for the primary, and 1400-1650 K and $0.05-0.07
M_{\odot}$ for the wide companion. For the $\sim$M6 component we estimated
$T_{eff}=2850\pm200 K$ and $m=0.10^{+0.06}_{-0.01} M_{\odot}$.
