In early form a. ONF. Jenever, Genever, = mod.F. Janvier: — L. Jānuārium, nom. Jānuārius (mensis), i.e. the month of <span style="font-variant:small-caps">Janus</small>, as presiding over the entrance into the year. Whence also It. Gennajo, Gennaro, Pr. Genovier, Januer, Sp. †Jenero, Enero, Pg. Janeiro. The later forms show gradual conformation to the L.; which was sometimes used unchanged in OE.
Wikipedia foundation.