Jastrow: הֶיסֵּת

הֶיסֵּת m. (יסת, v. וֶסֶת) consuetudinal law, equity; only שבועת ה׳ consuetudinal or equitable oath. [שבועת ה׳ is applied, if one who is sued for a debt, denies the latter entirely (כופר הכל), in contradist. to the legal oath which is required when the defendant admits a part of the claim (מודה במקצת). It being presumed that nobody will go to law unless he have a claim, it is a matter of equity to put the opponent to an oath, to which he may in return put the claimant.] Shebu. 40ᵇ; B. Mets. 5ᵃ; 6ᵃ