非分之想

Built around 非分 'not within one's portion/station'. A much-cited source is Lu Xun's 1934 essay 《花边文学·运命》 (Fate): 穷人买奖券，便是一种'非分之想' - a poor man buying lottery tickets is a kind of desire for what isn't proportionate to one's lot. Classical usage extends back further.

usage

Almost always used negatively, paired with 别有 / 不该有 / 不要有 ('don't have...'). Common contexts: a man pursuing a woman improperly, an employee eyeing the boss's position or assets, anyone reaching above their station.