From The Collaborative International Dictionary of English v.0.48:
Tab \Tab\, n. [Etymol. uncertain.]
1. The flap or latchet of a shoe fastened with a string or a
buckle.
[1913 Webster]
2. A tag. See Tag, 2.
[1913 Webster]
3. A loop for pulling or lifting something.
[1913 Webster]
4. A border of lace or other material, worn on the inner
front edge of ladies' bonnets.
[1913 Webster]
5. A loose pendent part of a lady's garment; esp., one of a
series of pendent squares forming an edge or border.
[1913 Webster]
6. a small projecting piece of a file folder, file card, or
similar sheet used in a filing system, on which a notation
is written to permit convenient search for the folder,
card, etc.
[PJC]
7. a bill or check for some purchase, as in a restaurant; as,
the salesman will pick up the tab.
[PJC]
8. a key on a typewriter or computer keyboard which advances
the carriage or curser to the next (preset) tab position;
-- used especially to type or print text or numbers in
columns.
[PJC]