EASY COME, EASY GO= This is an English proverb that means that the things that you get without any effort, are easy to lose because you don't appreciate them and you don't fight for them or take care of them. For example, if you work hard to save some money, you'll appreciate that money and spend it wisely, but if you win a lot of money in the lottery, you spend it all in silly things because you don't appreciate it. Another saying with a similar meaning is "no pain, no gain" (if you want to learn or obtain something good, you need to make an effort).

SHOULD OF KNOWN= This is a colloquial (or mistaken) spelling which is quite common because the auxiliary verb HAVE, when speaking, sounds /v/ after a vowel (I've got /aɪv gɒt/) and /əv/ after a consonant (I should've know /aɪ ʃʊdəv nəʊn/), and the pronunciation of OF is also /əv/, so it is easy to confuse the spelling of both words because they sound the same. (careful: /ɒf/ is the pronunciation of OFF!)

YOU WAS= (slang; black English or simply wrong English) You were.

TROUBLE= /trʌbəl/ Problems.

WIDE OPEN= Totally open.
Popular wisdom says, "never trust someone who kisses you with their eyes open". Why? Because if you love someone, when you kiss them you get lost in the kiss and you close your eyes to feel it more intensely, but if you don't feel anything, you may keep your eyes open, so if they kiss you with open eyes, they probably don't love you, and that is what he means in this song. That's why he asks WHY WERE THEY OPEN?

GAVE YOU= I gave you.
In colloquial speech and songs it is common to drop the subject in some cases where it is obvious.

TOSSED IT IN THE TRASH= Threw it away, rejected it.
To toss is to throw, and the trash (AmE) is the litter bin (BrE), the place where you throw the rubbish.

CAUSE= (coll.) Because.

GRENADE= (also "hand grenade") A weapon which is like a small bomb. You hold it in your hand, pull the ring and throw it at the enemy because in a few seconds it explodes. If they throw a grenade and you catch it, you will die because it's going to explode in your hand in an instant (so he says that he would catch a grenade to save her if it is necessary, because he loves her more than his live). (see picture)

YA= (coll.) You.

BLADE= In a knife, sword or similar weapons, the long and thing metal part which has a sharp and cutting edge to cut. The blade is attached to a handle, which is the safe part where you grab it. So if you take a knife, you hold it by the handle (usually wood or plastic) and you cut with the blade (usually metal).

I WOULD GO THROUGH ALL THIS PAIN= If you go through pain, you suffer the pain, you experience that pain.

BULLET= /bʊlɪt/ The small metal thing shot by a gun to kill someone (see picture).

BRAIN= The soft and greyish stuff inside our head that some people use to think.

YOU WON'T DO= You don't want to do, you refuse to do.
The verb WILL (and it's negative WON'T) is often use with the same meaning as "want". In that case, the past of WILL is WOULD:
- Will you come to my party? = Do you want to come to my party?
- I want her to be my partner, but she won't = but she doesn't want to.
- I asked her why she did it, but she wouldn't say = but she didn't want to tell me.

BLACK AND BLUE= If they hit you hard (for example in the eye), after some hours your skin will turn dark blue or black, so if you go black and blue, you show the consequences of a fight or an attack. (see picture)

BEAT= Hit (attack someone by hitting with fists or a hard object)

NUMB= /nʌm/ When a part of your body is numb, you can't feel anything there. For example, sometimes, after you have been sitting on the ground, your leg may go numb and you don't feel anything there so you have to move it and wait for the blood to circulate back through it before you can walk (and when circulation comes back you feel "pins and needles", an unpleasant itchy feeling).

I SAID "HEY"= I said "hello", I sent him my regards.
He's telling her to say hello to the devil when she goes back to where she came from, because she's living in hell (because she is a very bad bad girl).

MAD= (esp. BrE) Crazy.

IN MY FACE= In front of me.
The opposite is "behind my back". If you do something "in my face", you do it openly, so I know. If you do something "behind my back", you do it secretly, so I don't know. Some people pretend to be your friend and smile to you in your face, but then insult you behind your back, so they are not your friends, they are hypocrits.

RIP= Break (by removing).

BRAKES= (in a vehicle: car, bike, etc) /breɪks/ The device that lets you stop or slow down when you're driving. If they break your brakes of your car, you can't stop the car and you'll have an accident.

ON FIRE= Burning, in flames.

BURN DOWN= If something burns down, it is totally destroyed by the fire.

YOU NEVER EVER DID= We can say "never ever" (or "never ever ever...") To emphasize the word "never" (if we say "never never" we are using a double negative).