This interesting article came from Dictionary.com’s Hotword Blog. 

Is “Rocket ‘88” by Jackie Brenston and his Delta Cats the very first rock and roll record? The question has inspired debate among musicologists for years. Another equally contentious question: Where does the term “rock and roll” come from?

“Rock” is derived from the Old English roccain, related to the Old Nordic rykkja meaning, “to pull, tear, move.” The earliest recorded use of the term in literature can be found in the lullaby “Rock-a-bye Baby “ from 1805.

“Roll” is derived from the Latin rotula meaning, “small wheel.” The phrase “rocking and rolling,” a metaphor used by seamen to describe the motion of a ship, dates from the 17th century. Similar metaphors slipped into popular discourse, but one in particular became the inspiration for the genre’s moniker.

By the 1920s, “rocking and rolling” became a popular double entendre referring to either dancing or sex. Trixie Smith’s 1922 blues ballad, “My Man Rocks Me (With One Steady Roll) may be the first use of the phrase in song.

Alan Freed, a disc jockey in Cleveland, Ohio used the phrase, “The Rock and Roll Session” to describe the amalgamation of rhythm and blues and country music he played during his show. As his radio show gained popularity, so too did the phrase.

And why is the “and” sometimes written as ‘n? That’s called an apocopation – the omission of the final sound of a word.