The phrase builds on purée, a common French minced oath used instead of a stronger swear word, similar to saying darn or shoot in English. Adding de pomme de terre turns it into mashed potatoes, a deliberately long and ridiculous version that French speakers share as a joke exclamation. The clip credited to Oxmoz_ captures that full line in a single incredulous shout and has circulated as a short reaction sound, especially for moments of surprise, denial, and comic frustration where a clean, expressive outburst works better than actual profanity.