I first heard about this from a well-known David Letterman Top 10 list. Phrases like “Jumping the Turnstyle” or “Tethering the Blimp” which sound like they mean something sexual but actually don’t mean anything.
So, I was behind a family minivan this morning. It had a bumper sticker which read: “We <heart> Back Door Donuts” Okay then.
After Googling it, it seems to be a shop in Massachusetts - but still…
“Take Exit 69 onto Big Beaver.”
Wankel Rotary Engine
Dick’s Last Resort
Blew my wad.
summa cum laude
Actually, I think I can make it sound even dirtier:
“Take 69 to Big Beaver and get off.”
“Hoist by his own petard.”
No - that actually is dirty. I mean, I know these days it simply means “spent all my money” or thereabouts, but it’s origin is sexual…
I seem to recall a scene where someone said “conjugating irregular verbs” with an arched eyebrow, but can’t recall where…
Again - it isn’t sexual/dirty these days - it means “done in by their own scheming” or so. But didn’t Cecil explain that it’s origin has to do with farting? Or perhaps the bomb that is referred to as a petard - which would hoist you if you set it off too early - was nicknamed using the word for fart, petard (like that Fartiste fellow, Le Petomane…)
Arranging financing on a big SUV: paying for a hummer.
You misunderstand. Big Beaver Road is an actual road in Troy, Michigan. It intersects I-75, and the exit from the highway to Big Beaver Road is actually exit #69. Looky-looky.
Rather reminds me of Big Bone Lick State Park.
Dongle plug.
I don’t misunderstand. I live by it.
With the properly arched eyebrow, ANY thing you say can sound dirty.
Oh yeah, but “conjugating irregular verbs” just sounded good that way…
Hiking the Appalachian Trail.
Etymologically, “petard” derives for a word meaning “fart,” so it’s a little dirty to begin with.
There are phrases in French that seem very dirty if said right. “Changement de vitesses” is one example.
Especially if you’re the governor of South Carolina 
- but then, it does kinda become dirty at that point…
Riding bareback.