quarantine

Where it comes from

Quarantine ordinarily means separation intended to prevent the spread of disease or another danger. English quarantine came through French from Italian quarantina, a period of forty days, built on quaranta, forty. Mediterranean ports used isolation periods against plague; Venice formalized a forty-day regime in the fifteenth century. The word first named the period of isolation, then the restraint or place itself.

How the meaning developed

Modern quarantine need not last forty days, but its number remains embedded in the name.