Synonyms of the word

clandestine

• adj. Done or kept in secret, sometimes to conceal an illicit or improper purpose.
• adj. (freemasonry, of a person or lodge) Not recognized as a regular member.

cloak-and-dagger

• adj. Marked by menacing furtive secrecy, often with a melodramatic tint or espionage involved.

covert

• adj. (now rare) Hidden, covered over; overgrown, sheltered.
• adj. (figuratively) Secret, surreptitious, concealed.
• n. A covering.
• n. A disguise.
• n. A hiding place.
• n. Area of thick undergrowth where animals hide.
• n. (ornithology) A feather that covers the bases of flight feathers.

hole-and-corner

• adj. Conducted in secret; clandestine.

hugger-mugger

• adj. Secret, clandestine, sly.
• adj. Disorderly, chaotic, confused.
• n. (archaic) Secrecy.
• n. Disorder.
• adv. Secretly.
• adv. Confusedly, in a muddle.

hush-hush

• adj. Secret, not spoken of (or spoken of only in hushed whispers between insiders).
• n. secrecy; covert operation.

secret

• n. (countable, uncountable) Knowledge that is hidden and intended to be kept hidden.
• n. (uncountable) Something not understood or known.
• n. (archaic, in the plural) The genital organs.
• adj. Being or kept hidden.
• adj. (obsolete) Withdrawn from general intercourse or notice; in retirement or secrecy; secluded.
• adj. (obsolete) Faithful to a secret; not inclined to divulge or betray confidence; secretive.
• adj. (obsolete) Separate; distinct.
• v. (transitive) To make or keep secret.
• v. (transitive) To hide secretly.

surreptitious

• adj. Stealthy, furtive, well hidden, covert (especially movements).

undercover

• adj. Performed or happening in secret.
• adj. Employed or engaged in spying or secret investigation.
• n. A person who works undercover.

underground

• adj. (not comparable) Below the ground; below the surface of the Earth.
• adj. Hidden, furtive, secretive.
• adj. Of music, art, etc, outside the mainstream.
• adv. Below the ground.
• adv. Secretly.
• n. (chiefly Britain) An underground railway.
• n. (with "the") A movement or organisation of people who resist political convention.
• n. (with "the") A movement or organisation of people who resist artistic convention.
• v. To route electricity distribution cables underground.