“on the nail”别理解成“在指甲上”，真实含义是立即；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。

I could put the bill on my credit card, but if it's all right with you I'd rather we divvy it up here and pay on the nail.
我可以用信用卡付账单，但是如果你可以的话，我宁愿我们分摊，我当场就可以付钱。