Minecraft wiki entry for Zombie_Villager, associated text: A splash potion of Weakness thrown by the player, a dispenser, or a witch.
An area effect cloud of Weakness from a lingering potion.
An area effect cloud of Weakness from a creeper explosion.‌[Java Edition only]
A tipped arrow that was dipped into a Weakness potion.
A command that applies the Weakness effect, e.g. /effect give @e[type=zombie_villager] weakness 2000 9 true.