again I'm not sure about an expression, though I suppose the meaning must be something like 'care for each other'.

The context: A woman was abandoned by her husband, her girl friends try to console her, and one of them says:

"We've all been there. The picket fence routine is a lot harder than it looks. That's whay we've to watch each other's backs."

Thank you very much in advance, Luce

Yes Luce, that's exactly how I would understand it too. It is indeed an equivalent to 'care for one another' or 'look after one another'.
With maybe a slight emphasis on the notion of guarding from a danger ?
It definitely has the sense of protecting each other from danger. Someone can attack you from behind - so someone watches your back. You might hear someone say "I've got your back" which means "I'm making sure you don't get harmed." The harm, of course, is not usually a physical attack, but perhaps emotional harm. (Unless you're watching a crime show. Then it does mean "I'll make sure no one shoots you from behind.)

So to "watch each other's back" means they are all looking out for one another to try to make sure no one gets hurt.
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Thank you very much, Waiti and Grammar Geek,

that was very helpful. Emotion: smile

Have a nice day,

can proficients give more examples of metaphors , similarism or idioms.

especially in compositions they give tremendous effect to our writing.i try to use some of them in my compositions as well but with my poor knowledge i think i repeat myself with cliches.

to draw blood from stone, as cunning as a fox, eats like a pig,grow like a roofs of tree and etc.

i ll be obliged if you give me more such beauties of english

I just Googled "idioms" and got this site: http://www.idiomsite.com /

Seems like it might have some good stuff for you.
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thanks and i have found so many answers to my stupid question sorry (why hadnt i searched this site before i asked this Emotion: smile)