Respectfully Quoted: A Dictionary of Quotations. 1989.
Not worth a Continental dam had its origin about this time . It is not a profane expression. A dam is an Indian coin of less value than one cent and a Continental one cent was next to worthless when it took six pounds, or about thirty dollars to buy a warm dinner.
OLIVER TAYLOR, Historic Sullivan, p. 97, footnote (1909).
Other versions of this phrase include Not worth a Continental and Not worth a Continental Damn. While other writers do not include the Indian connection, they agree the phrase arose when Continental money became worthless toward the end of the Revolution. See Mitford M. Mathews, A Dictionary of Americanisms, p. 383 (1951).