pay up 和pay off有什么区别？

二者都有付清的意思，确实很难区别；唯一微妙的区别在于pay off 必须是有借贷行为的还清；pay up不是由借贷关系产生的付清，而是由于某种特殊关系而产生的付清。从牛津高级词典中查到的句子如下：
You must pay off your old loan before you can obtain a new one.
Each club member should pay up his membership money before the end of the season.

pay up全部付清

[口语]1) 偿付(索据)；付清(钱)；全部(或按时)付清(欠款)：例句: I had to ask him several times for my money, and he paid up in the end.
我不得不向他讨了几回账，他最后终于还清债了。2) 在压力下或经要求后付款，(在勒索或压力下)拿出钱来：例句: The gangsters used threats of violence to force the shopkeepers to pay up.
歹徒们以暴力相威胁，强迫店主把钱拿出来。

pay off偿清(欠款等),贿赂

1. 偿清(债务等)；付清(工资)：例句: He paid off the debt of his friend with the money raised by his writings.
他用稿费的收入偿清了朋友的债务。2. 发清工资解雇：例句: It is expected that another twenty will be paid off before long.
另外的20人估计将在不久被资遣。3. 惩罚；(对…进行)报复：例句: Walter will be paid off for his negligence.
华尔特将因他的失职而受到处罚。4. 有报偿；赚回；得到好处；收到效果：例句: It has been rather costly to install the machinery, but it should pay off in the long run.
安装这套机械代价很高，但从长远来看它会赚钱的。5. (为免遭威胁)付钱(给敲诈者)；收买(某人)：例句: The blackmailer had to be paid off. He was too dangerous.
只得向那个敲诈勒索者付钱了，他太危险了。6. [口语]得到预料的结果；获得成功：例句: Did your daring plan pay off?
你那大胆的计划收到效果没有?7. 使(船)转向下风：例句: The captain paid his vessel off before the wind.
船长把船转向顺风。The little vessel paid off from the wind.
小船逆风而行。8. 放松或放出(缆索、绳子等)：例句: The rope should be paid off.
应该把绳子放松。

pay off,
付清工资；债务
a. to pay (someone) everything that is due that person, esp. to do so and discharge from one's employ.
b. to pay (a debt) in full.
c. Informal. to bribe.
d. to retaliate upon or punish.
e. Nautical. to fall off to leeward.
f. to result in success or failure: The risk paid off
pay up,
全部付清；按要求支付
a. to pay fully.
b. to pay on demand: The gangsters used threats of violence to force the shopkeepers to pay up.

建议楼主多看英文释义。这样最接近原意。
Good luck

pay up是付款 没说还清
而pay off是还清债款
常跟短语：pay off the debt