Tevye in "Fiddler on the Roof" sings:

"They would ask me to advise them,
Like a Solomon the Wise.
"If you please, Reb Tevye..."
"Pardon me, Reb Tevye..."
Posing problems that would cross a rabbi's eyes!"

What does it mean to cross one's eyes?
moody_mWhat does it mean to cross one's eyes?
to contemplate a conundrum

However, it is not an established idiom; in fact, it may have been coined by the lyricist.
Literally, "crossed eyes" means that instead of looking straight ahead, a person's eyes point towards each other.
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Reb is short for Rabbi. Once rich, Tuvya will have the status of learned Talmudic scholar asking intrinsically paradoxical riddle like complicated questions that would cause the rabbi to become cross eyed, as part of Pilpul which I explain here: