An Etymological Dictionary of the German Language/Annotated/schlaff
schlaff, adjective, ‘relaxed, loose, indolent,’ from Middle High German and Old High German slaf (genitive slaffes), ‘relaxed, idle, impotent’; compare Low German and Dutch slap, ‘relaxed, impotent,’ whence Modern High German schlapp, retaining the Low German p, is borrowed. Gothic *slapa- is perhaps a graded form of the root slép, as lata-, ‘idle, lazy,’ is of the root lêt, ‘to omit’ (see laß). Old Slovenian slabŭ, ‘relaxed, weak,’ and Latin lâbi, ‘to glide,’ lăbare, ‘to totter,’ have been rightly compared with the primary Teutonic slapa-, ‘relaxed.’ See schlafen.