The characters combine to create a vivid picture: 身 (your body/actions) 不 (is not) 由 (determined by) 己 (yourself). Your actions are being controlled by something other than your own will.
身不由己 is more than just a phrase; it's a window into a core cultural perspective. While Western cultures often emphasize individualism and personal autonomy, traditional Chinese culture places a high value on collectivism, harmony, and fulfilling one's role within a group (family, company, society). This idiom is the perfect expression of the conflict that arises between individual desires and these collective duties. It's often used to justify or lament a situation where one must sacrifice personal wishes for the greater good, to maintain social harmony, or to fulfill an obligation to one's parents or boss. A Westerner might say, “My hands are tied,” or “I'm between a rock and a hard place.” These phrases imply a difficult choice or a temporary constraint. 身不由己, however, often suggests a more persistent, existential state of being subject to a larger system. It carries a heavy sense of resignation to fate or duty, a feeling deeply rooted in Confucian values of social roles and responsibilities. The famous saying 人在江湖，身不由己 (rén zài jiānghú, shēn bù yóu jǐ)—“Once you're in the world (the 'jianghu'), you can't act on your own”—perfectly captures this sentiment.
This idiom is extremely common in both formal and informal conversations. It's a go-to phrase for explaining a difficult situation with a sense of resignation.
The connotation is almost always one of helplessness and frustration, but it also serves as a way to seek empathy and understanding from the listener. By saying 身不由己, you are saying, “Don't blame me; I had no choice.”