You can think of a small card in someone's wallet with their name and an organization on it. Anyone can say they are on your side; this person can reach into a pocket and show it. That act of proof is what the word carries. Once the picture is clear, English uses it on people who have no card at all — the card becomes a way of saying there is no doubt about this one.
可以想成皮夾裡有一張小卡，上面印著名字和組織。誰都可以說自己支持你，但這個人掏得出東西來證明。這個「掏得出來」的動作，就是這個字帶著的重點。畫面一旦清楚了，英文也拿它去形容根本沒有卡的人：那張卡變成一種說法，用來表示「這個人絕對假不了」。