The story
From Anglo-Latin franciplegium, a Latinization of Anglo-Norman frauncplege ("free pledge"), a mistranslation of Old English friðborh ("pledge of peace"), which had the corrupted form friborh, which led to the Modern English term friborg, as if it were *freoborh ("free pledge"). See also friborg, which refers to the predecessor of frankpledge.
Etymology from Wiktionary, CC BY-SA 4.0.