'Soap' is a Germanic export to Latin — Pliny recorded 'sapo' as a barbarian loanword. Rome learned from the north.
A substance used with water for washing and cleaning, made by combining fats or oils with an alkali.
From Old English 'sāpe,' from Proto-Germanic *saipō, meaning 'soap' or 'resinous substance.' The word may have been borrowed from a Celtic or Finnic source, as Pliny the Elder attributed the invention of soap to the Gauls and Germanic peoples, calling it 'sapo' — a word he identified as a Germanic loanword into Latin. The ultimate origin is debated: some link it to PIE *seib- (to pour out, to drip, to trickle), while others
Latin borrowed 'sapo' (soap) from Germanic — one of the rare cases where a 'barbarian' word entered the language of Rome. Pliny the Elder noted in his Natural History (77 CE) that the Gauls and Germans made soap from tallow and wood ash, and that Romans considered it a foreign curiosity rather than a hygiene product.