Have Legs Meaning

(nautical) To have speed.

Example: 2007, Pete Williams, How to Turn Your Million Dollar Idea Into a Reality (page 68)
  Your plan will tell you if your idea has legs and is viable.
2011, Larry Garrison, ‎Wallace Wang, Breaking Into Acting for Dummies, The Gatekeepers
  The longer a film plays (which, in show business terms, means that the film has legs), the more the theater gradually earns from ticket sales.