A horn regarded as or alleged to be obtained from the unicorn.
The horns came from various animals, narwhals being the most usual, and were often mounted or made into a drinking cup. Taken in powdered form, the substance was popularly believed, especially in the Middle Ages, to protect against poisoning.
Late 17th century (in an earlier sense). From Italian alicorno unicorn, medicinal substance made from the horn of a unicorn, ultimately showing a variant or alteration of classical Latin ūnicornis.