at source

1. Where something originates. Our hope is that cleaning up the river at source will have a restorative effect on the local ecosystem. Do you think that reducing air pollution at source will have a big impact on the air quality in our area? We've got to address this issue at source before it affects the water supply at large.

2. Before a payment is made to one, as of a tax deduction. If the government didn't takes people's taxes out of their paychecks at source, it might never see that money again. You know, you can set up an automatic deduction at source if you want to ensure you are regularly contributing to your retirement fund. Why don't you just let the government take their money from you at source? You never have enough money to pay them back come tax time!