The back of the tongue is the most common single source.
Gum inflammation
Bleeding, inflamed tissue contributes.
Untreated decay or a failing restoration
Food traps and stagnation.
Dry mouth
Saliva is protective; less of it means more odour. Medications often contribute.
Non-dental causes
Sinus, tonsils, reflux, and some medical conditions. Worth considering if dental care does not resolve it.
What tends to help
Cleaning the tongue, cleaning between teeth daily, treating any gum inflammation, and addressing dry mouth. Mouthwash masks odour; it does not remove the source.
If a thorough dental work-up does not resolve it, that itself is useful information and points toward looking beyond the mouth.
What we can explain here — and what needs an exam
A page can inform you. It cannot examine you.
We can explain online
The common oral sources
Why mouthwash is not a fix
How dry mouth contributes
Determined after an exam
Whether decay or gum disease is contributing in your case
Whether a restoration needs replacing
Whether referral is appropriate
What to do next
An exam and cleaning identify most causes. Mention it directly — it is a routine thing to bring up.