An Etymological Dictionary of the German Language/lassen
lassen, vb., ‘to let, leave,’ from the equiv. MidHG. lâȥen, OHG. lâȥȥan, str. vb.; comp. AS. lœ̂tan, E. to let, Du. laten, OIc. láta, Goth. lêtan; the pre-Teut. form of the common Teut. root lêt, ‘to leave,’ is lêd (with lad as a weaker gradation, comp. laß). The only certain cognate in the other Aryan languages is the Lat. word lassus, ‘faint, languid,’ quoted under laß; hence ‘to relax, release,’ is probably the prim. meaning of the verbal stem. From this, MidHG. lâȥen, both simply and in compounds, evolved the meanings ‘to set free, omit, leave behind,’ &c., as in ModHG.