The general concept of "back" is often expressed via lexicalized
fingerspelling. You spell the word "back" but you drop the letter
"a" and only slightly form the letter "C" as the
handshapes move from a "B" to a "K."
This is a good version of the sign to use in statements like: "give
it back," "when are we coming back?" Another aspect of
this sign is that it tends to move backward (for the general concept) and it
is also directional -- which means you can aim the palm-side of your hand in
the direction of the person that will receive whatever it is that is being
given back (or in the direction that you are going back to).
#BACK:
(Note: Some people do the letters "BAK" but in general you should just do
two the "B" and the "K.")
The back of your body. (Touch your back twice.)
BACK (version)
You can use an "A" hand and move it backward over your shoulder a couple of
times to mean, "It is in the back," "Back of the line buddy," or "back
it up."
BACK (version)
Note: If you mean, "make a back
up of an electronic file" you should use a version of the sign "get in back
of" which is a version of the sign "behind" which is a derivative of the
sign "WITH."
(Sorry. Shoot me now.)