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fifth column

A clandestine subversive organization working within a country to further an invading enemy's military and political aims.

[First applied in 1936 to rebel sympathizers inside Madrid when four columns of rebel troops were attacking that city.]

fifth colum·nism (kŏləm-nĭz′əm) n.

fifth columnist n.

Indo-European & Semitic Roots Appendices

Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:

Indo-European Roots

Semitic Roots

The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.

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