give the finger

1. Make an obscene gesture by closing one's fist and extending one's middle finger upward, generally interpreted as fuck you. For example, Herb has a dangerous habit of giving the finger to motorists who cut in front of him. Although from about 1890 to 1920 this term was used in the sense of “disappoint or snub someone,” that meaning seems to have died out. [Second half of 1900s]