According to the CNRTL link, in 1877 it was used in the same way we would use the gerund in English, to refer to the action ("signifying the washing of hair") then in 1890 became the word for the product used in that process (shampoo) as well.
That seems to happen from time to time in French, when the gerund also becomes the name of an object involved in the process, as with "le parking".
Sometimes it happens in English too, like "building" and "booking".
That seems to happen from time to time in French, when the gerund also becomes the name of an object involved in the process, as with "le parking".
Sometimes it happens in English too, like "building" and "booking".