If someone says Wallaahy (swears by Allaah) and is lying but over something really stupid e.g. he says Wallaahy it wasn't me who ate the cake when it was him who ate the cake. Does he need to do a Kaffaarah? What is the difference between an oath and an unintentional oath? What's an oath? May Allaah reward you for your help Inshaa Allaah.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If a person swears to a lie, he has indeed committed a great major sin as the Prophet said: "The great major sins are: Associating partners to Allaah, cutting relations with parents [or being undutiful to them], killing a soul without a valid reason and swearing to a lie."

Ibn Qudaamah said: 'This swearing to a lie is called [in Arabic] ‘Ghamoos’ (literally to immerse) because it immerses the person in sin.'  

So whoever has sworn to a lie is obliged to repent to Allaah. As regards whether or not he is obliged to expiate for it, the scholars differed in this regard. For the answer, please refer to Fatwa 88592.

Also, please refer to Fatwa 83550 with regard to an oath that is not intended, like saying: “No, I swear by Allaah”, or “Yes, I swear by Allaah.”

Concerning the meaning of an oath, it means to swear by Allaah.

Allaah Knows best.