Left pedal is unscrewing itself?

Well this is odd.

My left pedal fell off on my way home from work a few days ago. Somehow it managed to unscrew itself from the cranks as I screwed it back in just fine - when I got home I checked the crank and there appeared to be no damage to the thread, I cleaned it out and applied new grease and it all seemed ok.

Then on the way home this evening something felt odd and I noticed it half out already.

(then when I got home I noticed someone had pinched my effing GT85)

What on earth is going on, and how do I fix it?

And no my cranks are on the right way round.

That is curious, as in "impossible"...

Just covering all the bases: to insert/do-up, you have to turn it anti-clockwise?

Yes it screws in by rotating it in the way you would if you were pedalling.

It may make a difference that I have a coaster brake on this bike, so occasionally perform some fairly firm backward gestures. But I've never had this in the 4 years or so of owning this bike.

Are you cycling backwards?

Yes.

By which I mean, no.

Also googling around appears to suggest I need to tighten the fucking thing on as tight as fucking fuck. Going to try that first.

Try another left hand pedal in there, could show if problem is pedal or crank. Of course it could be both.

Have you tried turning it off and on again?

Hand pedals is the problem, try using feet.

GT85 is not for pedal threads.

are they toes strap pedals ? or spd's ?

if the spindles are a bit tight then the straps catching on the crank arms every rotation can gradually loosen them when you are pushing or having a few attempts at getting into the toestraps
this happened to me when i was ghost riding a bike luckily another person behind me saw them drop off and let me know

this happened to me when i was ghost riding a bike

CSB.

in the past to stop things coming undone i have greased them then wrapped cotton thread around and around, put it back on the bike nice and tight.. job done.