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1. Literally, the ability to walk steadily on a moving ship. Don't worry, once you get your sea legs, you'll be able to walk around the deck no problem.
2. By extension, one's comfort level or familiarity with something. Of course you're scared, it's your first day of teaching! But you'll get your sea legs soon enough, don't worry.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
*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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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.
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.