foedus

Foedus, foederis is third declension neuter. The verbs that go with it are facere, icere and above all ferire: foedus ferire, "to strike a treaty", from the pig that was struck down when the oath was sworn. The other party takes cum with the ablative.

The trap is a genuine homograph. There is a second foedus, foeda, foedum, an adjective meaning "foul, filthy, shameful", declining like bonus and unrelated to the noun. The nominative and accusative singular are identical; everything else separates them, since the noun's genitive is foederis and the adjective's is foedi. A foedum foedus is a shameful treaty, and Roman writers enjoyed that far more than they should have. English takes federal, federation and confederate from the noun, all through the stem foeder-.