The word "lay," as defined by Wiktionary and every sane and rational individual ever, means:
The site further provides the following example of the proper usage of this transitive verb:"To place down in a position of rest, or in a horizontal position."
Which is exactly what I want to do when I hear folks say shit like, "I'm going to lay down." Is that so? Oh, you mean you're going to lie down? Well, then, fucking say that. Because Wiktionary confirms that the word "lie" means:"...to lay a body in the grave..."
My guess is some people remember that kiddie prayer from way-back-when which starts,"To rest in a horizontal position on a surface."
"Now I lay me down to sleep..."But check it: in that sentence, I lay me down. I place myself down in a position of rest, or in a horizontal position. Thus, that usage works. The other crap doesn't.
Even the expression "getting laid" works, if you're using it to describe the act of some other lucky person placing you in a horizontal position for purposes of sexual intercourse. Presumably, if one readily, frequently, and indiscriminately permits oneself to be placed in a horizontal position for purposes of sexual intercourse, one might be considered an "easy lay" (but not by me, 'cause I don't judge). (Much.) (Nah, J/K. To each, her/his own.) (Except when it comes to grammar, in which case you'd better toe the goddamn line.)
So no, you weren't laying down on the couch, you were lying down. But you did lay your fork down on the table when you were done eating that lasagne. (You know, assuming you were using a fork.) (And if you weren't—ick.)
For a more comprehensive, and far less rabid, take on the matter, click here. Or just take my word for it. Whatever you do, do not get these wrong if you've a pedant like me in your vicinity, as the poor unfortunate may lose control and...I shudder to think what might happen.
::shudders::

