Nedarim 25A

אמר ליה: כי משתבע ⁠— אדעתא דידן משתבע, ואנן לא מסקינן נפשין אשומשמני.

Rav Ashi said to him: When he takes an oath, he takes an oath based on our understanding, which is that of an ordinary person, and we do not entertain the possibility in our mind that he is referring to ants [shumshemanei]. Therefore, if he took an oath in that manner, it is assumed that he referred to people, like those that left Egypt.

ועל דעתא דנפשיה לא עביד איניש דמשתבע? והתניא: כשהן משביעין אותו, אומרים לו: הוי יודע שלא על תנאי שבלבך אנו משביעין אותך, אלא על דעתינו ועל דעת בית דין. לאפוקי מאי? לאו לאפוקי דאסיק להו לאיסקונדרי ואסיק להון שמא זוזי.

The Talmud asks: And does a person not take an oath according to his own understanding? There are times when one takes an oath with a particular stipulation in mind or intends a special meaning to his words. But isn’t it taught in a baraita: When the judges administer an oath to one who claims he paid a debt, they say to him: Know that we do not administer an oath to you based on a stipulation in your heart, i.e., you cannot claim that you are taking the oath based on a condition you have in mind. Rather, your oath is taken based on our understanding and on the understanding of the court. The Talmud clarifies: What does the phrase that they say to him: Based on our understanding, come to exclude? Does it not serve to exclude a case where one gave the debtor tokens [iskundarei] from a game, and in his mind he gives them the title of coins and takes an oath that he returned these coins, which is the truth based on his unspoken thoughts.

ומדקאמר ״על דעתינו״, מכלל דעביד אינש דמשתבע אדעתא דנפשיה!

The Talmud clarifies its question: And since the baraita says that the oath taken in court is: According to our understanding, by inference it means that a person commonly takes an oath according to his own understanding and the oath would take effect according to his intent. Therefore, such a practice must be specifically excluded when taking an oath in a court.