An Etymological Dictionary of the German Language/Annotated/Schlummern
Schlummern, verb, ‘to slumber,’ from the equivalent late Middle High German (Middle German) slummeren, slumen; compare Dutch sluimeren; Anglo-Saxon slûmerian, English to slumber, Anglo-Saxon slûma, Northern English sloom, ‘to slumber.’ The root (Alemannian šlûne, šlûre, ‘to slumber’) contained in these words appears in Gothic slawan (slawaida), ‘to be silent,’ in a curious divergent meaning, to which Middle High German slûr, masculine, ‘idling, idler’ (compare Schlaraffe), is also allied. The primary idea of the whole group is ‘to be quiet, inactive.’