HOT N COLD= Though it's not usually written like this, the conjunction AND is very often pronounced "an" or simply "n". If we want to represent this we usually spell it like this: hot 'n' cold, rock 'n' roll (using apostrophes to mark the missing sounds).

YOUR LAWFULLY WEDDED HUSBAND/WIFE= Notice the formula they use to marry two people. LAWFULLY means "recognized by law", "legally". Wedded is the past participle of "to wed", to marry.

YOU CHANGE YOUR MIND= If you change your mind you change your thoughts or decisions, you think differently.

PMS= Pre-Menstrual Syndrome. An array of symptoms that comes from a hormonal upset as a woman's body prepares to either become pregnant or expel an unfertilized ovary. The worst time to be around a woman because they get so sensitive and irritable. So here, she’s telling him that he always behaves like a woman with PMS.

BITCH (a rude word, coll.)= A malicious or spiteful woman (offensive). It may also be used for men (but not usually) to mean he’s weak and/or vile (hateful).

CRITICALLY= If you speak critically, you often criticize people (say bad things about them)

CAUSE (coll.)= Because. You can also spell this like: ‘cause, coz, cuz.

YOU’RE IN/OUT, UP/DOWN= In the chorus she’s using opposites to explain that he’s always changing his mind, but some of these opposites may also have a precise meaning. "To be in" is to be fashionable (doesn’t fit in this context though), but it may also mean "to be introverted" and to be out could be "to be extroverted", but it’s also "to be at home / to be outside", or simply a way of expressing you’re sometimes like this and sometimes the opposite. To be up and down may be the same thing, just using opposites, but considering he’s bi-polar, it makes a lot of sense to consider its other meaning: "to be down" means to be depressed, so by contrast here, "to be up" would mean to be high, to feel great (unnaturally great).

WE BREAK UP= We finish our relationship.

WE MAKE UP= We start a good relationship again after breaking up or fighting. When two children fight and they are angry with each other, parents can say, "come on, kiss and make up", as a way of making them be friends again. The "kiss" part is not always taken literally.

WANNA= Want to. In colloquial English, especially American English, a T may be lost after an N. WANNA is really common in conversational AmE, and also quite common in colloquial BrE.

USED TO= We use this verb to express the idea that something was true in the past but not now anymore.

TWINS= Two siblings (brother or sister) born at the same time. If they are exactly the same, they are called "identical twins".

IN SYNC= In synchronicity, in harmony. Having the same feelings, likes and thoughts.

A DEAD BATTERY= A battery is a device to store energy (for example, the two cylinders or the button you put into your MP3 player to make it work). When a battery is exhausted, it has no more energy, we say that "it’s dead".

USED TO LAUGH ‘BOUT NOTHING= We used to laugh about nothing, for no reason (but that doesn’t happen anymore).

YOU’RE PLAIN BORING= You’re really boring. We use the word PLAIN to express the idea that there is nothing more to it, so it gives emphasis to the adjective. If you’re plain boring, you’re boring and that’s it, no exceptions or nuances. But "plain" is used like this with a negative meaning, we never say, "you’re plain lovely"; we would say "you’re simply lovely".
If a woman is plain, she’s ugly. If you speak "in plain English", you speak clearly, so everybody can understand what you say.

GONNA= Going to. Here we can say exactly the same comments we used when explaining WANNA. Going to --> gon’ to (colloquial) --> N+T= N.

SOMEONE CALL THE DOCTOR= This is the way to construct the imperative when we don’t refer to anyone in particular:
- call the doctor! (= you should do it)
- let Peter call the doctor! (= Peter should do it)
- Somebody call the doctor! (=anyone can do it)
The doctor here refers to a psychiatrist, because this boy has psychological problems, not a health problems.

GOT A CASE OF...= I have got a case of...

BI-POLAR= This is a psychological syndrome (illness) consisting of an unstable state of mind. A bi-polar person may feel enthusiastic and happy and suddenly feel depressed, so they change moods continuously for no reason. At this point, we can see that the real problem here is not just him, but the relationship, because she’s not calling a doctor to attend him, she’s calling a doctor to attend the relationship (we have a case of love bi-polar). It is the relationship which is ill, because he’s changing his mind all the time and she can’t break up, so they both have a big problem.

STUCK= Past participle of TO STICK. If something is stuck, it can’t move. If you’re stuck in a situation, you can’t get our or react or change it.

ROLLER COASTER= A fairground attraction (AmE: amusement park attraction) where you sit on a kind of little train and the train goes at great speed up and down, swerving or even looping. See picture here. "We’re stuck on a roller coaster" means that we have this ill bi-polar kind of love (going up and down) but none of us is able to change it or stop it. That’s why she says "can’t get off this ride", which means, I can’t get out of the roller coaster car and have a normal life.

GET OFF= To get out of a public transport. You get in/out of a car, you get on/off a train, bus, ship, etc. The roller coaster is like a train, so we say get on/off.

RIDE= A trip on the roller-coaster. A RIDE is the act of travelling by car, train, bus, bicycle or horse. If you give me a ride, you take me somewhere by car. If the bus-driver tells you that a ride is two pounds, that’s the price you have to pay to get on the bus.