I do not agree that the first one is "excellent".

The word "ass" is considered by many people to be a very rude word.

Its British equivalent ("arse") is in fact considered by many to be a swear word.

It's one thing to teach foreigners slang, but it's quite another to teach them rude and/or offensive words and try to pass them off as "excellent idioms".

English learners, please be aware: the first expression listed above is extremely rude and you should not say it to anybody whom you do not wish to offend!

When learning a foreign language it is of course necessary to learn a certain amount of rude words to add to your vocabulary (even if you don't want to use them yourself, it is useful to at least be able to recognise them when others use them, and thereby be aware when native speakers are being rude to you).

So I don't see anything wrong with teaching swear words per se. However, I think it's misleading to introduce an expression that contains a rude word and try to describe it as an "excellent English idiom".

Even if you replaced the rude word "ass" with a milder word (such as "backside") it would still be considered extremely rude in polite circles to say to somebody "get off your backside".

I have personally never said that to anybody in my life, and I hope I never have to.

I would suggest that a more humorous (less offensive) version would be "get your rear in gear".

The reason I like that one better is because it contains no offensive words, and it also happens to rhyme, which makes it easier for learners to remember.

Personally, I would never say any of these expressions. I would just say politely "please can you hurry up?" and leave it at that.

I've never heard "right up your alley" before. 

Perhaps in the US they say it differently. In Britain, I have only heard "right up your street".

Thanks for these, Justin. Just to let people know - these are both examples of American English.

Here are the British equivalents, for anyone who might be interested:

1.  Get up off your (fat/lazy etc) arse!

'Arse' is fine old English word going back many centuries. It is ruder than the later American version, and can't be confused with a donkey.

2. Right up your street.

'Look, here's a TV programme about Victorian steam railways. That's right up your street. You love that kind of thing, don't you?'