means Operating in secret, often by disguising one's true identity, in order to investigate or observe without being noticed.

from A transparent compound of "under" (from Old English under, the same word still doing its work) and "cover" (via Old French covrir from Latin cooperire, "to cover over completely"). The literal sense is to act while kept beneath a covering — hidden from view. The figurative "in disguise, secretly" sense is the modern one, blooming in the 20th century alongside spies, detectives, and the bureaucracies that hire them.