(informal) To be acquainted with the normal process or procedure of a given activity or situation; to be familiar with how something is done.

(historical) A member of a secret antiforeigner political organization in the nineteenth-century United States.

(historical) The doctrines, principles, or practices of the Know-Nothings.

(idiomatic) (of information) given to another only when necessary.

Something that one needs to know; a piece of necessary information.

plural of know-all

Knowing everything; omniscient.

present participle of know better

The state of knowing nothing; ignorance. / A perverse admiration or embrace of ignorance.

The attitude of a know-it-all.