Guards or soldiers posted to watch for danger and alert others; can also mean anything that warns or signals a problem.
From Italian 'sentinella,' possibly derived from 'sentire' meaning 'to feel' or 'to sense.' The word traveled through European languages as militaries adopted the practice of posting watchful guards during the Renaissance.
In medicine, doctors talk about 'sentinel nodes' in cancer treatment—these are lymph nodes that act like the body's early warning system, just like military sentinels stood watch at castle gates to spot enemies first.
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