To Darken the Doorstep is when you go to someone's home and you cast a shadow over their doorstep. It usually used to describe the action of someone you know and haven't seen for a while unexpectedly turns up at your home or it's something you tell an unwanted visitor not to do ever again.
Sue: "OMG, you never guess who had the nerve to darken the doorstep this morning."
Pat: "Who?"
Sue: "Jo!"
Pat: "Jo, Jo who?"
Sue: " you know the one that coursed all that trouble at my party."
Pat: " Oooh! That Jo, haven't seen her around here for ages, did you invite her in?"
Sue: " No I did not! Told her to clear off and to never darken my doorstep again."
Named after the extremely unsafe 2 to 4 inch door sill at the doorway of every Mexican museum in Mexico City. Watch as beefy American tourists trip and fall flat on their fat fucking faces as the museum staff smirk and don't help them up.
Gossipy Bitch 1 (Sipping Pre-Noon Sangria On A Beach Patio): What's with Helen these days? She seems to have an extra bounce in her step at hot snake yoga every morning.
Gossipy Bitch 2 (Also An Alcoholic): Well, you didn't hear it from me, but she's been fucking that young hot pool cleaner, Brody. It's no wonder, she's been complaining for years that her husband Bob is hung like a Mexican Doorstep...
A guy or girl who is attractive enough that if they showed up at your doorstep, you would have sexual relations with them and not turn them away. But they are not attractive enough to where you would go out of your way to pursue them