What does the medical abbreviation SS with a line and two dots over it mean?

Answer 1

It means one-half.
the ss you describe means half.

Answer 2

one half
SS on a prescription means half, but it's rarely used these days because it's too easy to confuse with 55.
one half

Answer 3

ss with a line over it means half. It's rarely used these days because of the risk of confusing it with "55"