An Etymological Dictionary of the German Language/Annotated/Schlaf
Schlaf (1.), masculine, Schläfe, feminine, ‘temple,’ from the equivalent Middle High German and Old High German slâf, masculine; Schläfe is properly the plural of Schlaf, referring to both the temples (compare Latin tempora); Dutch slaap, ‘temple.’ In Anglo-Saxon þunwęnge, allied to Old High German tinna, Middle High German tinne and Old High German thinna-bahho, masculine, ‘temple,’ Middle High German tünewęnge, ‘temple’ (compare dünn), Old High German dunwęngi, Old Icelandic þunnvange, ‘temple.’ Beneath these similarly sounding terms lies the older Teutonic term for ‘temple.’
Schlaf (2.), masculine, ‘sleep, slumber,’ from the equivalent Middle High German and Old High German slâf, masculine; a verbal abstract from schlafen, Middle High German slâfen, Old High German slâfan, strong verb, ‘to sleep.’ This form is peculiar to Teutonic in this sense, and is wanting only in Old Icelandic, which has preserved sofa (Teutonic root swef, Aryan swep), primitively allied to Latin somnus, Greek ὕπνος; Gothic slêps, ‘sleep,’ slêpan, ‘to sleep,’ Anglo-Saxon slœ́p, English sleep, Anglo-Saxon slœ̂pan, English to sleep, Dutch slaap, slapen, Old Saxon slâp, slâpan. Compare also the derivatives with r, Old High German slâfarag, Middle High German slâfrec, slœfric, ‘sleepy,’ Old High German slâfarôn (and slâfôn), Middle High German slâfern, ‘to be asleep, get sleepy.’ With the Teutonic root slêp, ‘to sleep,’ appearing in these cognates, are also connected Modern High German schlaff and its Teutonic correspondences; hence the primary meaning of schlafen is probably ‘to be relaxed.’ For further references see under schlaff.