Aims. From the luminosity, effective temperature and age of the Hyades brown dwarf 2MASS J04183483+2131275 (2M0418), substellar evolutionary models predict a mass in the range 39-55 Jupiter masses (MJup) which is insufficient to produce any substantial lithium burning except for the very upper range >53MJup. Our goal is to measure the abundance of lithium in this object, test the consistency between models and observations and refine constraints on the mass and age of the object. Methods. We used the 10.4-m Gran Telescopio Canarias (GTC) with its low-dispersion optical spectrograph to obtain ten spectra of 2277 s each covering the range 6300-10300Å with a resolving power of R∼500. Results. In the individual spectra, which span several months, we detect persistent unresolved Hα in emission with pseudo equivalent widths (pEW) in the range 45-150Å and absorption lines of various alkalis with the typical strengths found in objects of L5 spectral type. The lithium resonance line at 6707.8Å is detected with pEW of 18±4Å in 2M0418 (L5). Conclusions. We determine a lithium abundance of log N(Li)=3.0±0.4dex consistent with a minimum preservation of 90% of this element which confirms 2M0418 as a brown dwarf with a maximum mass of 52MJup. We infer a maximum age for the Hyades of 775Myr from a comparison with the BHAC15 models. Combining recent results from the literature with our study, we constrain the mass of 2M0418 to 45-52MJup and the age of the cluster to 580-775Myr (1σ) based on the lithium depletion boundary method.