Located or occurring outside of a serous membrane (the thin layers that line body cavities).
From Latin 'extra' (outside) + 'serous' (relating to serum, from 'serum' meaning whey or watery fluid). A medical anatomical term.
Your heart, lungs, and organs are wrapped in slippery serous membranes, so when doctors spot an extraserous problem, they know it's somewhere unusual—maybe lodged outside where it shouldn't be, which can mean trouble.
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