pilgrim

From Middle English pilegrim, from Old English pilegrī̆m, from Old French pelegrin, from Latin peregrīnus, meaning 'foreigner.' The shift from /r…r/ to /l…r/ happened through dissimilation in early Romance languages, as seen also in Italian pellegrino.

The facts here don't trace a documented shift in meaning beyond the sound change itself — but that change alone shows how a word for 'foreigner' softened, syllable by syllable, into the one we use for a traveler on a sacred journey.

It's a doublet of peregrine — both words descend from the same Latin root, peregrīnus, but took different paths into English.