Idaho is the state most famously said to resemble an extended middle finger on the map.
Its long, narrow north-south panhandle (the "North Idaho" region) projects upward between Washington (west) and Montana (east), while the broader southern portion forms the "hand." The analogy works best on standard Mercator-projection maps oriented with north up.
This shape resulted from historical boundary negotiations: the 1846 Oregon Treaty set the 49th parallel as the US-Canada border (Idaho's north), the 1863 Idaho Territory creation, and the 1868 Wyoming Territory creation (straight southern border). The panhandle's isolation from the rest of Idaho by mountains fostered a distinct regional identity.
Other states with "rude gesture" reputations: Florida (sometimes called "the gun" or "the finger" depending on orientation), Michigan (the "mitten" — friendly wave). But Idaho owns the middle finger comparison.