1
general::
The ability to adjust to a new situation or difficult conditions, as in She's only spoken in public a few times; she hasn't found her sea legs yet. This expression was first recorded in 1712 and then referred to, as it still does, the ability to walk steadily on board ship, especially in rough seas. By the late 1800s it was being transferred to other challenging situations.
American Heritage Idioms
2
general::
Phrase(s): *one’s sea legs
Fig. one’s ability to tolerate the movement of a ship at sea. (*Typically: get ~; have ~.) • Jean was a little awkward on the cruise at first, but in a few days she got her sea legs and was fine. • You may feel a little sick until you get your sea legs.
McGrawhill's American Idioms And Phrasal Verbs