the last mile
US /ðə ˌlæst ˈmaɪl/
UK /ðə ˌlæst ˈmaɪl/
1.
the final stage of a journey or process, often the most difficult or challenging part
:
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Completing the project was tough, especially the last mile of debugging.
•
Getting the product to customers is often the last mile challenge for logistics companies.