Answer: Taking the long walk
Lady is carted off to the pound while she and Tramp are escaping from an angry farmer, and she is locked up with two of Tramp's friends, Peg the Pekingese and Bull the bulldog, as well as a borzoi called Boris, a dachshund called Dachsie, a chihuahua called Pedro, and a mongrel called Toughy. Nutsy, another dog, is being led out by one of the workers at the pound and Toughy says he is 'being taken for the long walk'. It doesn't take long for Lady to figure out what that means.
Incidentally, Peg's song, 'He's a Tramp', is my favourite song in the movie. It is sung by Peggy Lee, who also voices Darling, and both Siamese cats.