6. Slang. to be very good, impressive, exciting, or effective: This show really rocks.
So where did this odd usage originate?
I think that rocks actually came about even earlier as a verb than the 1950s. According to the Oxford English Dictionary, by the 1920s rocks was a sexual euphemism in the US. In 1931, it started to mean:
Then, just a few years later, in 1938 a related meaning was added:
The earliest uses of the word in this sense are:
Rocks was first used to refer to jazz, and it was already being used to describe something with "the energy and drive characteristic of such movement". However, the OED writes that the slang usage emerged in the 1960s:
The first known use in the form is:
So the term originated with jazz, and took off from there.
It comes from the sense of rock 'n' roll music. If it's good, it rocks.
And from Etymonline:
If something has a strong beat, it is/was considered to be favorable or exciting.