One year while handing out candy, a kid who looked about ten years old came to the door wearing a shirt that said ‘this is my costume’. Needless to say, I gave the kid a tootsie roll. What is the laziest Halloween costume you’ve ever seen?
I’ve seen kids trick or treating in their street clothes, so I daresay that’s not even to be considered a costume. But it is lazy.
But for laziest costume, I had a pretty low-effort get-up one year. I didn’t have a whole lot of time to get a costume together ahead of time, so the afternoon before a family party, I got my hands on a “Hello, My Name is…” sticker. I wrote my son’s name on it and went to the party as my son in 32 years.
My company claims 90% participation in costume wearing on Halloween. I don’t cotton to such bullshit so I plan to go dressed as a telemarketer. Or, if I want to put no more effort into it whatsoever, a once-and-future engineer reduced to being a telemarketer for now.
I think the former might be the better choice.
You could be like Wednesday Addams and go in your street clothes. When asked what you are you reply “I’m a homicidal maniac, they look just like everyone else.”
A former girlfriend of mine once went as a witch. She was a Wiccan. (And no, she wasn’t making any kind of statement, she just blew off getting/making a costume until it was too late, so she declared her regular clothes to be a costume.)
Ripped a pice of my t-shirt once and went as James Dean.
Were you James Dean in mourning, or something?
I’ve had a few greedy teenagers try to pull the “serial killer” bit. (Like Wednesday Addams’ homicidal maniac: they wear their regular clothes and, if pressed, tell you they’re a serial killer.) My stock answer is that I’m dressed as a Jehovah’s Witness, then, and don’t give out candy.
Most of my costumes-for example two years I wore a suit and then tied those red wristbands they give around on Red Ribbon Week around by elbow and claimed I was a Communist revolutionary for Halloween.
When I worked fast food as a teen and worked one Halloween, three of my coworkers and I got the Dymo label maker out and changed our name tags to be the four horsemen of the apocalypse. Famine and Pestilence were on register, and War and Death were on grill.
Would have been funnier with Famine on grill. 
Made a last-minute decision to go to a co-worker’s Hallowe’en party. Wore my regular clothes. Several-block walk through the rain to get to his flat. When asked about my lack of costume, I pointed at my wet blue shirt and blue jeans and said, “I’m a lake.”
A few years ago I grabbed a pair of sunglasses and a gray hoodie and went as the Unabomber.
Nah, better to have Famine on register asking if you want to super-size it.
When I was 13, I decided last minute that I wasn’t ready to give up trick or treating, so I decked myself out in black, head to toe, and told people I was an invisible pedestrian, ala SNL. That was the last year I went door to door.
Sorta harder to get around these days, I imagine, what with the quadriplegia and all. 
mmm
Towards the end of the evening, we always see bunches of teens in no costumes, just begging for candy. The Grinch in me would like to snarl at them, but I don’t want them throwing eggs at my windows, so I give them candy anyhow. Sometimes, I make a crack like,
“Nice costume, you really do look like an orc.”
I embarrassed the heck out of one young man (who I actually knew as he’s the son of a friend; I don’t think I’d do it to a stranger) by gushing, “Oh, my gosh! Jenny! I love your costume! You look just like Nick!!! Jenny, what did you do with your boobs, Jenny? Did you bind them? Are you wearing a corset? Has Nick seen your costume? We have to take a picture, 'cause this is the BEST COSTUME EVER!!!”
He wore a costume the next year.
WhyNot: Parenting With Sarcasm Since 1993
I find when people wear jeans and a sports jersey quite lame.
Just sayin’… 
My lazy costume was a couple years ago when I woke up the morning of the Company Costume Party (this was Purim, in Israel, but pretty much the same idea…) with no costume.
So I threw on a pair of sneakers, sweats, a cap and picked up an old tennis racquet I had laying around, et voila!
The kicker of course was that this was very shortly after Muhammad Mabhouh was offed in Dubai, supposedly by Mossad agents dressed as tennis players*
So I was really dressed as a Secret Agent, dressed as a Tennis Player!
Best low-effort costume EVAH!!
Oh, another even lazier one – I’m a red-head, a real carrot-top. So on year I simply ducked into the men’s room just before the party and sprayed my hair black. Got everybody doing a double-take on that one.
*From the wikipediapage about the incident
(bolding mine)