Writing in Newspaper Days about the Great Fire of Baltimore in 1904, H.L. Mencken described his staff at the Herald as going to work “with the enthusiasm of crusaders shinning up the walls of Antioch.”
To shin or, more colloquially, shinny up or down an object—tree or pipe, whatever—is to climb by hugging it with the arms and legs. The verb derives from the noun shin.
But when one encounters a description of the act today, it is most frequently labeled as shimmying, which is a grave mistake. To shimmy derives from the dance of that name, popular in the Twenties—a kind of foxtrot in which the dancers shook their bodies. It has also come to mean wobbling of the front wheels of an automobile or, colloquially, a chemise.
If it will help you to maintain the distinction, to shinny is something that Tom Sawyer or Huck Finn would have done in climbing a tree. To shimmy is the action that Blaze Starr would have performed on the stage of the 2 O'Clock Club on the Block in Baltimore, endearing herself to Earl Long, governor of Louisiana and brother of the slain Huey, their tendresse memorably recorded by A.J. Liebling in The Earl of Louisiana.