Cassidy (song)
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"Cassidy" is the closing track from Bob Weir's 1972 Ace album.
Lyricist John Perry Barlow composed the lyrics to the song about two people, Neal Cassady and Cassidy Law (the daughter of Eileen Law, one of the Grateful Dead's archivists). Some of the lyrics also relate to the death of Barlow's father.
The song became a part of the Dead's repertoire, but it wasn't performed until March 23, 1974, which would be the only time they would ever perform the song with one drummer. After their return to touring in 1976, it became a regular part of their setlists.
Barlow himself has supplied a very thorough history and explanation of the song at this site.