In the beginning, around the middle of the sixteenth century, there was the word slip-shoe, about which there is nothing mysterious. It was simply a shoe that one could easily slip on or off, one that English speakers even then also called a slipper.
A little later in the same century, a person who wore a slip-shoe began to be described — naturally enough — as slip-shod. Within a few decades, however, it began to take on the negative associations that have remained with it down the generations. A person who was described as slip-shod was wearing shoes that weren’t suitable for polite company because they were literally down at heel, shabby, over-loose or untidy.
Our modern meaning of some activity that was lacking in care, badly organised or slovenly came about in the nineteenth century. Writers were the first to suffer its disopprobrium, with critics describing what they felt was “slipshod English”; the wider sense grew out of this.
Most of us have forgotten — or never knew — the connection between slip-shod and those comfortable sixteenth-century slip-on shoes, whose shabbiness and unfashionableness has bequeathed the language a useful term.