I don't understand some sentences clearly, could someone help me? Thanks in advance.

1. " I swear on my mother's grave. "------ Why on one's mother's grave?
2. What's does " 3-ly method " mean?
3. They go out and about. ------ Does it mean go out to lounge? Is "about" a preposition?Is it correct used here?
4. He is as nutty as a fruitcake. -------Does it mean the man is tough or weak?
Hi, lindamao Emotion: smile

1. I'm only guessing here, but I think it's perhaps because one's mother is considered sort of "sacred", so if you swear on her grave it will mean that you are telling the truth and/or being very serious about what you're saying.

2. I have no idea about this one.

3. As far as I know, to be "up and about", when speaking of someone who has been ill, means that the person in quesiton is able to get up and leave the house. I don't know if ithe expression has any other meanings. Both "out" and "about" are adverbs here, not prepositions.
"up and about" has a similar meaning: to be out of bed and able to walk after having been ill.

4. It means he's completely mad! ~L~

I hope it helps.
To go "out and about" typically means to leave the house for a casual trip around town to do things such as visit friends, shop, etc.
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About number 1, right guess Miriam.

About number 2, any context to help us discover what does it mean, Lindamao? I've never heard of it.
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I saw it only itself, no context included, on a phrase training material writen by an American teacher. I have the teacher's email, I will write to ask him sometime.
Thank you all.
ok, Lindamao.

If you get to know the right meaning of this expression, please post it!

I am curious..