“on the nail”别理解成“在指甲上”，正确的含义是立即，马上；当场。

【例句】
They’re good customers who always pay on the nail.
他们是很好的顾客，总是立即付钱。

She'd turn up to court to pay their fines, cash on the nail.
她出现在法院，当场就用现金支付了他们的罚款。