The mackerel is not exactly a noble fish. So a racing mackerel - a mackerel who's swimming faster than the others - is a person who thinks he's a big shot but, in fact, is a nobody.  

Apparently, Portuguese men have an appetite for small women, as well as for small sardines. 

A forgetful person. Silly!

Mills are usually located and powered by rivers. The water that has passed the mill will not make it work again. So this means that you shouldn't worry about things of the past. 

A person who threatens by talking a lot is usually harmless, like the dogs that bark. You can also use it to mean that a person who claims to do a lot usually does very little.

This means that character traces or personal skills must be learned from a very young age. This has something to do with the curvature in the cucumber, that is supposed to exist from the cucumber's early age. I think that Portuguese cucumbers are more curved that the American ones...

You say this of a color you cannot easily describe. Like, your eyes have the color of a donkey on the run. Try not to say this to your loved one...

The Moors were the Muslim people from North Africa who lived in the Iberia Peninsula, and they were the archenemies of the catholic Portuguese (and Spanish). They were "a threat" for a number of centuries. This expression, however, has nothing to do with war. On the contrary, it is related to love. You say this when there's a person threatening to invade someone's heart.

Portugal suffers from the small-country-with-a-big-neighbor syndrome. The Spanish (from Castilla, to be precise) are the other stereotypical enemies of the good people in Portugal. Several times there were battles to either prevent them from invading the kingdom or to kick them out. We're friends now, but the echoes of the past can still be heard. 
Apparently, East winds (from Spain) bring storms. Likewise, Spanish spouses bring trouble.

This is way over my head or this is more than I can handle. I would say this, for example, of quantum mechanics.

Don't mix friendship with business. Portuguese have a funny habit of repeating words, in this case "friends, friends". If you know anyone who's Portuguese, I'm sure you've caught him/her in a typical "yes yes" or "no no".

If you do something wrong you'll pay for it in this lifetime and in the place where you did the wrong thing. The "they" refers to the "bad things".

The kind of friends you have tells a lot about the kind of person you are.

I don't give a damn. Actually, I'd like to know where the hell this idiom came from... no clue!

If you don't behave, I'll slap you. "To eat" is literal of "comer", which in Portuguese means both "eat"-as-of-food and being the target of physical attack - "to eat a slap".

This is a warning in the form of an ironic negative. When someone is in trouble and does nothing to get out of trouble, he/she is trusting that the Virgin [Mary] will come up with a miracle. As opposed to be doing something to get out of trouble. So this warning means that you should do something instead of waiting for a miracle.

Buzz off!

This is not a good day, but said in a weird way. As if there are "yes days" (good, positive days) and "no days" (bad, negative days).

You don't know what you're talking about or you're making a fool of yourself. This means that you're disgracing yourself, you're letting water in - like a boat that's about to sink.

You say this when you're intrigued or surprised with something. As if being bitten by a monkey would bring you back to reality.

This is mostly used when talking to children, when they do something they're not supposed to do. The translation looses a lot of it, because it can't capture the masculine "bad". Yes, it's supposed to be masculine "bad" followed by the female name "Maria", which, of course, is weird.

He doesn't say or do anything right. I have no clue of where "to give [] for the box" comes from.

"To sand" here means to do a slight harm. So you can also say "Go sand yourself!" ( buzz off! )

What a waste! You say this of a person who has a wonderful opportunity and doesn't know what to do with it.

A pearl of popular wit! You say this of a person who is allocating resources without making good use of them. A person who can't make decisions. I know a few like that...

This may be typical of my region only (Beira Alta), because many Portuguese people have told me they never heard this. But I did, many times. "Arrimar" means both to hit someone and to hit on someone. This idiom usually comes when you mention some female cousin of yours, and you're a man. Like "Dorothea is my cousin." And the person you're talking to says back, with a naughty look, "The more the cousin, the more you hit her". 

This is not Portuguese at all, it's Brazilian. But I love to say it, because I'm a meat eater. It truly captures the spirit of red meat...

In case of doubt... or just in case...

You say this when you're mad or intrigued about something.

This is a friendly warning, something like you should watch out or else... "Putting a stick on yourself" captures the bodily movement of standing straight, like a stick. 

A "donkey" is dumb person. This idiom means that whatever is being said is stupid and dumb and you don't care, because you're in a much higher level - in heaven, to be precise. 

If a "donkey" is dumb person, an old donkey is even dumber. So he can't learn anything new, like languages. People say this about themselves, as excuse to not wanting to learn something new.

This means that when the essential goods are missing, people get upset, argue with each other for whatever reasons, and no one makes sense. The root of the problem is the lack of those essential goods.

At night, you can't distinguish objects and people too well. It's easy to make mistakes.

There's no such thing as perfection. "if" (senão) is used here as a noun to express the strings attached to any good thing, the conditions without which such beauty can't exist.

It's ok to be afraid - we're all afraid at some point, we're all humans. The interesting thing is the association between having the asshole and being afraid. I guess this refers to the fear of being raped and sodomized. If you didn't have the asshole such act wouldn't be possible!