“To be honest…” / “If I’m being honest…”
So, what, you’re normally lying?
“It is what it is”
A tautology is a tautology.
“Tell me how you really feel” (said after you share a harsh opinion on someone / something)
Didn’t I just do that?
“I’ll hold down the fort”
Why? Is it threatening to float away?
I’ll admit that last one is a bit David Mitchellian.
Not the person you replied to, but I’m guessing that they’re a stickler for the original meaning, which is “to reduce by a 10th”, hence the “deci” in decimate. It comes from a military discipline practice that could only have been invented by the Romans. The force to be punished is divided into groups of 10, and each group is given a bag containing 9 white pebbles and one black pebble. They each take turns reaching into the bag blindly and grabbing a pebble. The people with the white pebbles are then each given a cudgel and ordered to bludgeon the poor bastard with the black pebble to death, hence reducing their overall numbers by a tenth, or decimating them.