Waiting for the penny to drop

[Ninurta]

LOL, I am waiting for the hammer to drop.

We all know it is coming.

Wait a sec: Hammer to fall ... or ... drop the hammer?

Is this one of those Mandela Effect / Berenstain Bear moments?

I think that may be a distinctly American idiom, but both are correct, under slightly different circumstances.

"Waiting for the hammer to fall" refers to the lock time on old black powder guns, I believe. Gun springs were hand made in those days, and some were faster than others, meaning some fire locks had longer lock times than others. If you got a bad piece with bad springs, you might have to hold steady for a bit to let "the hammer drop" to the ignition pan, particularly in the times of matchlocks. One might have to hold his breath for a bit for a steady hold between the time the trigger was pulled and the time the ball actually left the muzzle, most of that time being taken up during the time the hammer took to drop the match into the flash pan.

"Drop the hammer" also refers to firearms, but in the context of someone whose days are numbered, or some action being ordered to be taken - "drop the hammer on him" means to launch a projectile at the offending party. "Drop the hammer on it" means to initiate a course of action concerning whatever "it" might be.

During the days of matchlocks, what we now call  "hammer" was actually called the "dog". Through flintlocks into cap locks, and on to modern firing mechanisms, the name gradually changed from "dog" through "cock" to finally "hammer", likening it's percussive effects to those of the tool bearing the same name.

.