"Du gehst mir tierisch auf den Keks!" - You’re extremely annoying.
'Tierisch' (like an animal) is generally a great colloquial way to spice up 'very'. You can also say “Du gehst mir (tierisch) auf den Geist!” which probably makes it clearer that your mind suffers from someone's obnoxiousness, but I guess the cookie gets more points for whimsicalness. I can't explain it, but I guess most Germans would get upset if you walked over their cookie.
"Du kannst mich mal!" - Fuck you.
This one is interesting, because it’s very vulgar without actually using bad language. Standing alone it’s most commonly understood to be an elipse of “Du kannst mich mal am Arsch lecken.” (You can lick my ass.), but with the right pausing you can follow it up with basically anything and still get your point across. Very common and therfore offensive even without a pause is “Du kannst mich mal gern haben.” (You can like me.) A variation on this theme is “Du kannst dich ins Knie ficken.” (You can fuck a [implication: my] knee.) which has the additional perks of actually using bad words and threatening violence.
"Jetzt weiß ich, wie der Hase läuft." - I see how the wind blows.
When you know how the bunny runs, you’re familiar with a complicated system. You know how things are done, you understand the rules. Most commonly the implication is, that the system is not in your favour: “Ich hab eh keine Chance, ich weiß doch, wie der Hase läuft!” Apparantly knowing how the bunny runs won’t help you catch it.
"Nun mach’ aber mal halblang!" - Come off it!
Everyone who ever heard a rant can probably relate with the wish for it to be only half as long. And sure enough you can use this phrase when you think someone is over-reacting. Apart from that it’s probably in the top ten phrases German parents say to their teenagers.
"Wieder was gelernt." - [We have] learned somthing again.
I’m actually unsure how to translate this one. It’s the kind of comment that can be used in different contexts and can mean different things depending on inflexion. There’s the resigned sigh of that sentence, probably when someone actually tried to teach you something you can’t be bothered with, the accomplished one when you’re done with a museum, there’s the intrigued version when someone told you a cool fact or a saucy bit of gossip, there’s the sarcastic one when you watched a stupid movie or the dead-pan one when you accidentally walked in on your co-workers fucking on top of the photo copier. I guess we really do have a slight obsession with the concept of life-long learning, because a comment on wether you have learned something is an acceptable and unsurprising reaction no matter what you have done.
"Nur die Harten komm’n in ‘n Garten."
I’m not sure this one actually means anything. I think it used to be a radio ad for a club in Berlin (where presumably only the hard ones were allowed into the garden), and it sure enough comes up when people are drunk and either something hard or a garden is mentioned. It’s either macho or just ridiculous. There’s a rather tortured pun in it, but I don’t think there’s ever actually a point to it. An adequate reaction to it is to have another beer.
"Hör’ auf mit dem Herumeiern!" - Stop dithering!
There’s really not much to it. It means you should get your shit together and decide already. To make a guess about what eggs have to do with it: apart from being undecided or vague about your plans ‘herumeiern’ is most commonly the action of riding a bike very badly, probably because it makes people think of the somewhat crooked path a rolling egg takes.
"Das macht dir so schnell keiner nach!" - Dude, you’re awesome!
It literally means that you did something nobody else is going to repeat in the near future. It is usually a genuine compliment. Just note that it’s very easy to make it a backhanded one, because it’s also true if you did something so dangerous and/or stupid that no sane person is going to try it again.
"Wie geil ist das denn?" - How awesome is this?
'Geil' is basically the German equivalent of 'awesome'. Just note that it also does mean 'horny', so you substitute it with 'fantastisch' or 'großartig' in a more formal setting.
"Jetzt haben wir den Salat." - The shit has hit the fan.
Not as graphic a picture, I’m afraid. A ‘Salat’ (salad) is something that would be hard to sort out, so it sometimes refers to a metaphorical mess. And the problem is on the table now.
"Nu (ma) Butter bei die Fische." - Let’s get this shit done!
This sentence is a grammatical nightmare and a somewhat nonsensical prompt to do/finish something (and do it right this time). I guess the idea is that there’s some fish to fry (and there’s an interesting metaphor-overlap) and the way to do it is get the butter and, you know, do it.
"Ich glaub’ ich spinne." - No way!
This one is more accurately translated with “I think I’m crazy.” Like its English counterpart the German verb ‘spinnen’ has a metaphorical meaning. You can spin a yarn (like spiders do) and you can spin a yarn (like seamen do), so it’s only Bing translator that’s getting confused over arachnids. The phrase itself is very common and useful to register astonished disbelief. Apart from that you get the entire spectrum of emotions, from anger at another driver who did something unbelievably stupid to joy over meeting an old friend in an unlikely location.