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, ˜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 ± 0{^''.}01 (˜30 au). We find the proper motion of SDSS 2131-0119 of μαcos δ = -100 ± 20 mas yr-1 and μδ = -230 ± 20 mas yr-1 consistent with the proper motion of the primary provided by Gaia DR2: μαcos δ = -95.49 ± 0.96 mas yr-1 and μδ = -239.38 ± 0.96 mas yr-1. Based on optical and near-infrared spectroscopy, we classify the primary NLTT 51469A as an M3 ± 1 dwarf, estimate photometrically the spectral type of its close companion NLTT 51469B at ˜M6, and confirm the spectral type of the brown dwarf to be L9 ± 1. Using radial velocity, proper motion, and parallax, we derived the UVW Galactic 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 a 670.8 nm Li I absorption line, and absence of H α emission, detected X-rays, or UV excess, indicate that the system is likely a member of the thin disc population and is older than 1 Gyr. For the parallactic distance of 46.6 ± 1.6 pc from Gaia DR2, we determined luminosities of -1.50^{+0.02}_{-0.04} and -4.4 ± 0.1 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 ± 0.02 M⊙ for the primary, and 1400-1650 K and 0.05-0.07 M⊙ for the wide companion. For the ˜M6 component, we estimated Teff = 2850 ± 200 K and m = 0.10^{+0.06}_{-0.01} M⊙.