he early uses of the term 'putting a foot in the door' are straightforward literal ones. It may just describe someone who steps over the threshold of a property, or someone putting a foot in the door in order to prevent it from closing and so continue a conversation. An early example of the latter comes in the American poet and playwright George Boker's work Plays and poems, 1856:
"And he sang to his gittern of love and of war With one foot in his stirrup and one in her door."
We now use 'foot in the door' in a figurative sense, with a similar meaning to 'the thin end of the wedge'. It was the technique of jamming a foot in the door to prevent it closing, used by door-to-door salesmen and political canvassers, that gave us this figurative use of the term. All the early examples are from the USA,