APAD: The penny drops.
Meaning:
A belated realization of something after a period of confusion or ignorance.
Background:
The Oxford English Dictionary states that this phrase originated by way of
allusion to the mechanism of penny-in-the-slot machines. The OED's earliest
citation of a use of the phrase with the `now I understand' meaning, is from
The Daily Mirror August 1939:
And then the penny dropped, and I saw his meaning!
The image of someone waiting for a penny-in-the-slot mechanism (which often
jammed) to operate does sound plausible and, if that isn't the origin, it is
difficult to imagine what is.
- www.phrases.org.uk [edited]
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My 20s and 30s were spent in confusion: too many open questions and things that
I didn't know that I didn't know. Into middle age, however, it felt like pennies
started to drop left and right. It's hard to know where to begin.
Monday, Jorkey the Spaniard asked me to show him the trap-and-roll from the
turtle bottom position facing the opponent. I learned it from Henry Akin's
videos three years ago and have since enjoyed taking 200+lbs guys down
regularly.
As the guy on top tried to grab my chest, I trapped his arm under my armpit and
put up my opposite leg as usual. His 140lbs was too light, however, to trigger
my knee-jerk reaction to pull him forward by digging my heel into the mat. I
naturally did a wrestling sit-out and rolled him directly onto his back. No
pulling at all. I never thought it could be so effortless. A giant penny just
dropped!
(The picture shows not exactly what I did. It just gives an idea.)