An Etymological Dictionary of the German Language/Annotated/fasten
fasten, verb, ‘to fast,’ from the equivalent Middle High German vasten, Old High German fastên; compare Gothic fastan, Old Icelandic fasta, Anglo-Saxon fœstan, English to fast, Dutch vasten; a common Teutonic verb, invariably used in the sense of ‘to fast,’ which, therefore, was probably a religions conception even of the heathen Teutons. The corresponding abstract is Gothic fastubni, Anglo-Saxon fœsten, Old Saxon fastunnia, Old High German fasta, fasto, masculine, Middle High German vaste, feminine, vasten, neuter, ‘fast,’ whence Slavonic postŭ, ‘fast, was borrowed at an early period. The cognates are probably connected with fest in the sense of ‘to contain oneself, exercise restraint in eating and drinking,’ or ‘to obey a religious precept’; compare Gothic fastan, ‘to adhere to, hold, observe.’ —