An Etymological Dictionary of the German Language/Annotated/ja
ja, adverb, ‘yes,’ from the equivalent Middle High German and Old High German jâ (for jă); corresponding to Gothic ja, ‘yes,’ also jai, ‘truly, forsooth,’ Old Saxon ja, Anglo-Saxon geâ, also gese (for gê-swâ, ‘yes, thus’), whence English yea and yes. Allied also to Greek ἢ, ‘forsooth,’ and Old High German jëhan, ‘to acknowledge, confess’ (see Beichte). Lithuanian ja is derived from German.