Learn English – Why do you survive ‘by the skin of your teeth’

From the New Oxford American Dictionary:

cross one's fingers (or keep one's fingers crossed): put one finger across another as a sign of hoping for good luck. • hope that someone or something will be successful.

The origin of this hand gesture is discussed, for example, in Wikipedia. It is not fully clear, but probably comes from pre-Christian times, and thus does not have a relation to the Christian symbol of the cross.

"No skin off my teeth" is not an expression in the UK. Apparently it does exist in the US where it means "no skin off my nose".

Saying "X is no skin off my nose" means that X occurring won't affect you in a positive or negative manner, the outcome will be neutral to you. The phrase is explained by phrase finder as being of boxing origin. I assume because boxers' noses are the body part most prone to damage.

As James McLeod points out "skin of my teeth" means only just or barely. This question addresses its origin.