An Etymological Dictionary of the German Language/Felsen
Felsen, m., ‘rock,’ from the equiv. MidHG. vęlse, vęls, m., OHG. fęlis, m., felisa, f. (from which Fr. falaise, ‘cliff,’ is borrowed); akin to OSax. fęlis, m., probably also to OIc. fjall, ‘mountain’; the latter would be *filza- in Goth., the former *falisa-; in Du. and E. the word is wanting. OIr. ail (from *palêk), ‘rock,’ OSlov. planina, ‘mountain,’ Sans. parvata, ‘rock, mountain,’ may be primit. allied. Connected also with OInd. púr, ‘fastness, citadel,’ to which Gr. πόλι-ς has been referred? or with Sans. pâšâṇa (for *palsâna), ‘stone’?.