An Etymological Dictionary of the German Language/Annotated/Rätsel
Rätsel, neuter, ‘riddle, perplexity,’ from the equivalent Middle High German râtsal, rœtsel, neuter, Old High German *râtisal, neuter; compare Old Low German râdisli, Middle Low German rêdelse, Dutch raadsel, Anglo-Saxon rœ̂dels (for *rœ̂desl), masculine, whence the equivalent English riddle, the s of the Anglo-Saxon word being regarded as a sign of the plural. The formation of the substantive from raten corresponds to that of Mühsal from mühen, of Labsal from laben, and of Trübsal from trüben. The notion ‘riddle’ was current among the Teutons from early times; the Gothic term was frisahts; in Old High German we find tuncal, neuter, and râtussa, râtissa, feminine ‘riddle.’