sea legs
sea legs {n. phr.}
1. Adjustment to being in a boat that is rocking on the sea.
This is my first transatlantic trip so give me a day to get my sea legs before you make me dance.
2. Adjustment to a new job or situation.
"I have just been transferred here and I haven't found my sea legs yet," the new colleague joked.
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Source: A Dictionary of American Idioms