Scarlett went off Thursday with Arianna Huffington at the Women in the World Summit in NYC ... eviscerating Ivanka after Ivanka said her biggest influence will happen behind the scenes.

The actress was baffled, saying her approach of standing behind a man and only voicing her disagreements in private "is so old fashioned and it is so uninspired and actually really cowardly."

Scarlett was a big Hillary supporter and recently mocked Ivanka on 'SNL' with a fake perfume ad called "Complicit."