The story
Latin: pactum (“agreement”) + dē (“concerning”, “about”) + nōn (“not”) + petendō (“which is to be sued for”, the ablative singular of the neuter substantive of the future passive participle of petō, “I demand or claim at law”, “I bring an action to recover”, “I sue for”) ≅ “agreement not to sue”.
Etymology from Wiktionary, CC BY-SA 4.0.