know backwards

[Itasan]

Is this understanding correct?
1. know something backwards - BrE
2. know something backwards and forwards - AmE
Do we need '-s' (backwardS) in both cases?
Does it mean 'know everything'?
Thank you.

[tigertiger]

BrE requires 's' if - to know something backwards
Possibly as something backwards is primitive, or of low intelect - e.g. The society is a bit backward (primitive) e.g.2 he is a bit backward (stupid).

However
to know something inside-out and backward(s) - to know very well- the 's' is not always used.

[Itasan]

Thank you very much, tigertiger.
The first-hand information from the native
speaker is really really valuable.
Thank you.

[Dodger]

Is this understanding correct?
1. know something backwards - BrE
2. know something backwards and forwards - AmE
Do we need '-s' (backwardS) in both cases?
Does it mean 'know everything'?
Thank you.

Greetings, Itasan.

I wish to inform you that the information which you seek concerning the above is as follows:

"backwards and forwards."

It is a fixed expression. The meaning is that one knows something so well as to be able to recite it from beginning to end as well as in the reverse, although only figuratively.

Sorry, dear friend, but not all native English speakers share the same understanding.

[Itasan]

Thank you very much, Dodger. US?

[Dodger]

"Thank you very much, Dodger. US?"

I am an American, living and teaching in Switzerland.