The earliest shops were nothing like the impressive buildings of today. They were small stalls or booths, like the ones in present-day markets or like traders' improvised set-ups in an arcade or alleyway.
And that is what the word shop originally meant?'a booth, shed, or stall'. In 16th-century slang shop was also used to mean 'a prison', and so to shop someone was to put them in prison. That's where the modern slang sense of shop, 'to tell the police about someone's criminal activities', comes from. The more orthodox use of the verb shop, meaning 'to go round the shops buying things', did not come in until the 18th century.
