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Great song by Nancy Sinatra! Still makes me somewhat confusin...
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Included only for reference (no need to read the whole song)
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You keep saying you've got something for me.
something you call love, but confess.
You've been a messin' where you shouldn't have been a messin'
and now someone else is gettin' all your best.

These boots are made for walking, and that's just what they'll do
one of these days these boots are gonna walk all over you.

You keep lying, when you oughta be truthin'
and you keep losin' when you oughta not bet.
You keep samin' when you oughta be a changin'.
Now what's right is right, but you ain't been right yet.

These boots are made for walking, and that's just what they'll do
one of these days these boots are gonna walk all over you.

You keep playin' where you shouldn't be a playin
and you keep thinkin' that you´ll never get burnt.
Ha!
I just found me a brand new box of matches yeah
and what he knows you ain't HAD time to learn.

These boots are made for walking, and that's just what they'll do
one of these days these boots are gonna walk all over you.

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Actually I understand the whole song.

You've been a messin - Have been doing something
oughta be truthin - You should say truth
keep samin - being the same
shouldn't be a playin - should not play
oughta be a changin - you should change
oughta not bet - you shouldn't bet on something


What is confusing about it is that these are not idioms which are more or less rational.

(by the way, I think I expressed my self clear, but maybe not formally right. I would be greateful if
someone writes the right version of this sentence)

It's more like to be some kind of slang or something. E.g. "keep samin" this is defenitely not an idiom. Ok not defenitely, but according to dictionary.com (http://www.reference.com/search?q=keep%20samin&r=d&db=web ).

You've been a messin (What indefinite article 'a' is doing here?)
oughta be truthin - (What 'in' is about?)
keep samin - (What is that, I mean how do we get it?)
shouldn't be a playin - (What indefinite article 'a' is doing here?)
oughta be a changin - (What indefinite article 'a' is doing here?)
oughta not bet - (I guess "oughta" means ought to?)

The main question is: How I'm supposed to figure out what such phrases mean?
(to my own surprise I coped with those above)

P.S. Hope you've got enough patience to read my post and still willing to help me! ;-)
Thank you in advance!
 
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You just have to READ more. The only way to meet more such patterns. 

a messin = a-messing
a isn't an article, which allows transforming the verb in an adjective here

You've been a messin - Have been doing something
Wrong. It means you have been messing/meddling/interfering where you shouldn't have.

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KhoffActually, I think you've done remarkably well in understanding the words as much as you have! Song lyrics are often not standard English.

I second what Khoff said - good job!

Except perhaps I would change "are often not" to "are rarely"!
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Comments  
> You just have to READ more. The only way to meet more such patterns. 
Right. But still the only way to understand the meaning of such pattern at first time I see it is guessing... Which isn't much exciting, cause you never can be sure of yourself.

Or maybe asking someone who knows (like you Emotion: smile for e.g.)

> a messin = a-messing
> a isn't an article, which allows transforming the verb in an adjective here
Ok. But what is it then?

> You've been a messin - Have been doing something
> Wrong. It means you have been messing/meddling/interfering where you shouldn't have.
I don't get why "where you shouldn't have" part is there?

Cause if it should, then second part (bold) of the line:
You've been a messin' where you shouldn't have been a messin'
is practically useless, cause it's already been said in the first one.

I would interpret it as merely: "you have been messing/meddling/interfering" and that's it.
You're right; these are not ordinary idioms. The words that end in "-in" are really "-ing" forms where the final "g" is not pronounced. (In some instances in the lyrics the "g" is replaced by an apostrophe, in others it's just omitted) And the "a" is not an article -- it's "an archaic or dialectic form" that used to be used before the participle ("-ing" form). It's often found in old somgs or poetry -- "I love to go a-wandering...") It has no meaning; you can just ignore it when it preceds a verb in the -ing form. So the lines you quoted mean:

You've been messing (interfering in something that's not your business)

ought to be "truthing" (telling the truth)

keep "saming" (doing the same thing)

Shouldn't be playing

ought to be changing

ought to not bet.

Actually, I think you've done remarkably well in understanding the words as much as you have! Song lyrics are often not standard English.
Students: Are you brave enough to let our tutors analyse your pronunciation?
 BarbaraPA's reply was promoted to an answer.
khoff, outstanding explanation!
Thank you very much! Emotion: smile

I guess, Marius Hancu is right: I really should read more!