"Truckin'" was originally a dance move. There are several theories about where it came from, but the most likely is that it was invented in Harlem during the
late 1920's. It was
done to a shuffle rhythm and involved moving away from your partner while strutting and waggling your index
finger.
It was popularized in the
late sixties by cartoonist Robert Crumb. His "Keep on Truckin'" cartoon in Zap comics (1968), featuring a guy leaning way back with his index finger up and his foot thrust forward. It was a
popular subject of poster art in the
late sixties.
The Grateful Dead recorded a song called Truckin' in 1971. By then, the term was pretty much over, so the Dead had nothing to do with it. Eddie Kendricks recorded a song called "Keep On Truckin'" in 1973. Hot
Tuna, a Jefferson Airplane spinoff, also recorded a (different) song under the same name. "Truckin'" was often a euphemism for a similar word with which it rhymes.