Idioms and Phrases with seeing things
Experiencing hallucinations or delusions, as in I thought I saw my father, but I must have been seeing things; he died twenty years ago. [First half of 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.