1
general::
Perceive flashing lights, especially after a blow to one's head. For example, A swinging door hit me and I really saw stars. [Late 1800s]
American Heritage Idioms
2
general::
Phrase(s): see stars
Fig. to seem to see flashing lights after receiving a blow to the head. • I saw stars when I bumped my head on the attic ceiling. • The little boy saw stars when he fell headfirst onto the concrete.
McGrawhill's American Idioms And Phrasal Verbs