Recorded in 1970, this song was remixed for Coda, an album of unreleased tracks that came out after the group's drummer John Bonham died. It was intended for Led Zeppelin III, but didn't make the cut.
Jimmy Page and Robert Plant wrote this at Bron-y-aur, the cottage in Wales where they wrote most of Led Zeppelin III.
In the song, Tom finds out his wife is cheating on him and shoots her. It is highly atypical for a Zeppelin song: jug-band, acoustics, and harmonica. More of the skiffle than the Hammer of the Gods here, albeit still unmistakably Zeppelin.
Our resident Shakespearean scholar thumped his dusty "Complete Works" tome on the conference table to verify that The Tragedy of King Lear does, indeed, have a character in it called "Poor Tom." The name is first introduced in Act II, Scene III, "The open country," as the assumed alias of Edgar, son of Gloucester, who has been an established character previously. Edgar is driven mad by an attempt to usurp his rightful heritage, so adopts the name as a partial defense, but really his madness makes him a likely companion for Lear. Wait... Where are you going?
Anyway, the "Poor Tom" reference gets passed through John Steinbeck's East of Eden along the way, and this song appears to be a more direct reference to a line from Steinbeck.
The lyric "seventh son" is a sideways reference to the myth that the seventh son of the seventh son is born with special powers, which could include being psychic, fortunetelling, having unusual skills, being able to talk to mustard in the fourth dimension, etc.