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I assume that your question refers to the "very knowing American" of Swift's acquaintance in London. It is important to be aware of the contextual reference here. Note that this is the paragraph wherein Swift finally presents his "modest proposal," that children should be eaten to provide for the hungry in the famine that Ireland was experiencing and as another commodity to raise money. To Swift's readers, the label "American" would suggest a barbaric person, therefore somebody who was uncivilised and would definitely come up with such a horrendous scheme. However, note the way in which Swift repeatedly tries to present himself as a careful and prudent economic planner by referring to the opinions of others to back up his proposal. The fact that he should refer to an "American" as one of his expert witnesses makes his proposal all the more abhorrent.