The answer to 'my supply line smells bad' is that a foul odor coming from your plumbing supply lines is usually caused by bacterial growth or a leak. This can happen when water sits stagnant in the lines, allowing bacteria to thrive and produce unpleasant smells. It could also be a sign of a small leak that is allowing water to pool and attract bacteria.
If the smell is mild, you may be able to resolve it with some simple DIY steps like flushing the lines or using a disinfectant. However, if the odor is strong or persistent, it's best to call in a professional plumber to properly diagnose and fix the underlying issue. They can inspect the supply lines, identify the source of the problem, and make any necessary repairs or replacements to get your plumbing system back in good working order.