you never can tell

Also, you never know. Perhaps, possibly, one can't be certain, as in You never can tell, it might turn into a beautiful day, or You may yet win the lottery—you never know. The first term uses tell in the sense of “discern,” a usage dating from the late 1300s; the variant dates from the mid-1800s.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.