it is ad idiom. it meas that it is in the past and you let it go. Similar phrases are "forgive and forget" and "letting it go". It basically means that a person did something that they failed to do.
USA
Let It Slide was created in 2006.
Let Your Backbone Slide was created in 1989.
The Bible
stop
from joey in friends
latin
1820-30
from girls ...to hissy...
in concord and lexington.
facebook entry
Andy Maio invent this phrase in the early 70s