I remember it as "think", at least when I hear the original recording. He's probably remembering the "do" from "What do I do when my love is away?".
The first stanza seems a bit out of context with the rest of the song though. I think it's a reference to "Billy Shears" (played by Ringo in the song) joining the band and seeming "out of tune" or "out of key" with the rest of the Beatles.