Unclear, but reflects an earlier ablative form. Proposals include:
* from Proto-Italic *jougestos (“yoked”), from *jougos (“team of yoke animals”) (whence Latin iūgera pl), from Proto-Indo-European *(H)yewg-es-, from *(H)yewg- (“join together”);
* from Proto-Italic *jugistos (“closest”), equivalent to *(H)yug- + *-istHos (“superlative suffix”); however, this suffix is otherwise not found in Latin.
The accusative is from the suffix used adverbially.