The Collaborative International Dictionary of English v.0.48:

Tam-o'-shanter \Tam`-o'-shan"ter\, n. [So named after Tam
   o'Shanter, a character in Burns's poem of the same name.]
   A kind of Scotch cap of wool, worsted, or the like, having a
   round, flattish top much wider than the band which fits the
   head, and usually having a tassel in the center.
   [Webster 1913 Suppl.]