Negative keywords can be used at the campaign or ad group level to restrict the irreverent search queries for which the product ads are shown. You have an option to add, edit, or remove negative keywords by clicking the Keywords tab.
How to find negative keywords?
By using Google Analytics, you can navigate to Acquisition > AdWords > Search Queries. Use Secondary Dimension > Advertising > Campaign and from the advanced filter, look for search option to segment the Shopping Campaign and apply the filter.
This report lists all matched queries for shopping campaigns. The report also includes behavior analysis from which you can see bounce rate. Click on the bounce rate to sort from 100%. Here you will see keywords that are irrelevant to the business. You can also compare the range month wise so that you can get a clear insight on selecting the right negative keywords. Adding those negative keywords would save a lot of $$$$ and this should be a continuous process. In the same section find out the converting keywords and re-optimize the most converted product pages with those relevant keywords.
Take advantage on both negative keywords and converting keywords!