1
general::
Go away, as in Get lost, we don't want you around. This rather rude slangy imperative dates from the 1940s.
American Heritage Idioms
2
general::
Phrase(s): get lost
1. to become lost; to lose one’s way. • We got lost on the way home. • Follow the path, or you might get lost. 2. Inf. Go away!; Stop being an annoyance! (Always a command.) • Stop bothering me. Get lost! • Get lost! I don’t need your help.
McGrawhill's American Idioms And Phrasal Verbs