Short answer
No. Swearing, lying, backbiting and other sins of the tongue do not break the fast in any of the four schools, so you do not make up the day. But they are haram, worse in Ramadan, and they can strip the fast of its reward.
The fast is valid and there is no qada. Lying, cursing, obscene speech, backbiting and gambling are haram at all times, and doing them while fasting is a heavier sin that ruins much of the reward of the fast.
The four schools agree on this.
Explanation
The fast is broken by eating, drinking, intercourse and a few things that enter the body. Words do not enter the body, so the fast of someone who lies or swears is still valid in law. The four schools agree. A few early scholars held that backbiting breaks the fast, but that view is not followed.
The fast is meant to train the tongue. The Prophet ﷺ said that whoever does not give up false speech and acting on it, Allah has no need of his leaving his food and drink. He also told the fasting person not to speak obscenely or shout in anger, and if someone insults him or picks a fight, to say: I am fasting.
So the scholars speak of two things: a fast that is valid, which you do not repeat, and a fast that is accepted and rewarded. Lying, cursing, backbiting and gambling leave the first and lose much of the second.
- Cursing or swearing at someone: does not break the fast. Repent, and apologise if you hurt someone.
- Lying: does not break the fast, but it is among the worst things a fasting person can do.
- Backbiting: does not break the fast. See what backbiting is.
- Gambling: haram in itself, whether or not you are fasting.
If you slipped, your fast stands. Make istighfar straight away, and if you wronged someone, put it right. For other things that weaken the fast without breaking it, see what is makruh while fasting.
The evidence
Quran, Surah Al-Hujuraat 49:12
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ ٱجْتَنِبُوا۟ كَثِيرًۭا مِّنَ ٱلظَّنِّ إِنَّ بَعْضَ ٱلظَّنِّ إِثْمٌۭ ۖ وَلَا تَجَسَّسُوا۟ وَلَا يَغْتَب بَّعْضُكُم بَعْضًا ۚ أَيُحِبُّ أَحَدُكُمْ أَن يَأْكُلَ لَحْمَ أَخِيهِ مَيْتًۭا فَكَرِهْتُمُوهُ ۚ وَٱتَّقُوا۟ ٱللَّهَ ۚ إِنَّ ٱللَّهَ تَوَّابٌۭ رَّحِيمٌۭ ﴿١٢﴾
O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy or backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allah; indeed, Allah is Accepting of repentance and Merciful.
Backbiting is a serious sin, likened to eating the flesh of a dead brother.
Sahih al-Bukhari 1903
Sahih... من لم يدع قول الزور والعمل به فليس لله حاجة ...
"Whoever does not give up false speech and acting on it, Allah has no need of his leaving his food and drink."
Lying does not break the fast but empties it of its purpose.
Sahih al-Bukhari 1904
Sahih... فإن سابه أحد أو قاتله فليقل إني امرؤ صائم ...
"If someone insults him or fights him, let him say: I am a man who is fasting."
The fasting person holds his tongue even when provoked.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Sawm, chapter on what breaks the fast and what does not. The fast is broken only by what enters the body or by intercourse, not by sins of the tongue.
- al-Majmu' (Nawawi), Kitab al-Siyam. Backbiting does not break the fast in the view of the scholars generally, though it lessens its reward.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.