The term “con man” is short for “confidence man”.
The first known record of its use was to describe New York serial criminal William Thompson.
“Operating in New York City in the late 1840s, a gently-dressed Thompson would approach an upper-class mark, pretending they knew each other, and begin a brief conversation. After initially gaining the mark’s trust, Thompson would ask “Have you confidence in me to trust me with your watch until tomorrow?” Upon taking the watch (or, occasionally, money), Thompson would depart, never returning the watch.
Thompson was arrested and brought to trial in 1849, in a case that made newspaper headlines across the country. The New York Herald, recalling his explicit appeals to the victim’s “confidence,” dubbed him the “confidence man.”