pirate, thief
戝 centers on a person who takes valuables by force or stealth: a pirate or thief. Both meanings share the idea of illicitly seizing treasure or goods.
戝 combines 貝 (shell, representing valuables or money) and 戈 (halberd, weapon), suggesting taking valuables by force. The exact historical development is uncertain, but the component combination fits the pirate/thief meaning.
Picture a pirate holding a weapon 戈 next to a pile of treasure 貝: that's a thief stealing valuables.
For ザイ, imagine a pirate shouting 'ZAI!' as he grabs the treasure: ZAI -> ザイ, the sound of a thief seizing loot.
—