In an interview with ELLE, Melanie Martinez stated that the “Soap” is about being vulnerable, and also explained its background:

Everyone is allowed to be vulnerable. I think women and men and dogs and cats and ants and aliens can all express themselves and be vulnerable. […] ‘Soap’ was written about my current boyfriend when we were first talking. I felt too scared to say how I felt about him and thought if I told him it’d be like throwing a toaster in his bath. So I washed my mouth out with soap. I think anyone can really relate to this song. I’m sure there was a time in everyone’s life where they felt too scared to say how they felt so they ‘washed their mouth out with soap.’

“Soap” is the second single from Melanie’s debut studio album Cry Baby. Its original music video was released on July 10th, 2015, and a revamped version premiered alongside “Training Wheels” in a double-feature video on November 18th, 2015. “Soap” appeared on the soundtrack of the 2016 film Nerve.

The story and illustration for this song appears on the thirteenth page of the Cry Baby storybook, reading:

She met a new boy and filled with hope
She said too much and always choked
So she washed her mouth out with soap
So that he wouldn’t pull the rope