To understand why it means that, we must first understand the difference between lay and lie.
The difference between lay and lie is like the difference between sadism and masochism: one means doing something to someone (or something) else while the other means doing it to oneself.
So, if we think of it that way, then lay is the sadist-because we use it when we are putting or placing someone or something else down.
Please just lay the leather whip down by the handcuffs.
See? The leather whip is being placed down.
Lie, on the other hand, is when one places oneself down.
She told him to lie down on the floor and bark like a dog.
In this case, he reclined himself.
So, the reason that people who want to “get laid” are lazy is because laid is the past tense of lay. Therefore, it means they want someone else to do it to them.
Here's an example of when to use laid:
Yesterday, Carl laid Susan on the bed.
(Get your mind out of the gutter; she fell asleep watching Letterman, and Carl picked her up off the couch and laid her on the bed.)
Now, the thing that makes this all very complicated is this: guess what the past tense of lie is?
I know- it’s totally screwed up right? It's like some weird, messed up relationship, like when your nemesis turns out to be your father:
Or the woman you are in love with turns out to be your sister:
Or your daughter is also your sister:
Anyhow, it's weird and complicated, and whoever was behind that decision was definitely a sadist.
So, we would write
Yesterday, Fluffy lay around all day, so he better not lie around today too.