Err on the side of caution is an idiom that means to plan ahead and calculate carefully rather than taking a risk or making a mistake.
The phrase “err on the side of caution” was derived from the meaning of the word “err” meaning ‘to make a mistake’.
“Err” comes from the Old French word “errer” from circa 1300.
The word “err” first appeared in print in 1678 in the book Of the Heart, and its Right to Soveraign written by Thomas Jones.
The idiom was used most frequently throughout the 17th – 18th century, but it is still part of everyday language.
As for its use in the 20th century, it has started to become more popular since 1986.