This is an odd expression, to be sure : 'It's between you, me and the lamppost ' What is its origin ? It seems to be universally understood in Anglophone countries, odd though it is.

I've also heard variations with 'gatepost' and 'doorpost'. I guess the idiom pre-dates streetlighting.

Apparently, the phrase wouldn't be understood in Singapore. In 'Singlish', an unwanted third person accompanying a romantic couple is a 'lamp post'.

"Expressions & Sayings" gives this possible origin of the phrase:

"Between you, me and the gatepost - between ourselves, in confidence.
Formerly '... and the bedpost', which made better sense in implying intimacies as of people in bed together. Perhaps as the expression passed round among people who knew nothing of four-poster beds they substituted a sort of post that was more familiar to them. However, 'post' had long been metaphorical for anything deaf, lifeless or ignorant; what matters is not the type of post but the fact that any post can be relied on not to reveal a confidence."

Expressions & Sayings

PS - I too have heard it said various ways.