All the people all around thinkin she was my chick
Sayin, "Damn, the girl thick, but she ain't no twenty-six"
Everyone who hung out with him assumed that due to her ubiquity and knowledge of his likes/dislikes (e.g. food) she was his girlfriend. They were also quick to state that although she was very voluptuous, they knew he was many years her senior (i.e she was younger than 26, presumably his age at the time)
To help improve the meaning of these lyrics, visit "Birthday Girl" by The Roots (Ft. Patrick Stump) Lyrics and leave a comment on the lyrics box