The phrase "old truism" refers to a commonly known and accepted idea or saying that has been around for a long time. It is usually considered to be true and holds wisdom from the past. Full definition
Populism has rocked the political culture of the West, overthrowing old truisms. (firstthings.com)
It is, after all, an age - old truism that power corrupts. (inc.com)
In fact, even the old truism about key simplicity no longer holds in this particular BMW, where the ignition is started by pushing a button on the dash. (canadianbusiness.com)