The Collaborative International Dictionary of English v.0.48:

Skirret \Skir"ret\, n. [A corrupted form equivalent to
   sugarwort.] (Bot.)
   An umbelliferous plant (Sium Sisarum syn. Pimpinella
   Sisarum). It is a native of Asia, but has been long
   cultivated in Europe for its edible clustered tuberous roots,
   which are very sweet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Sium sisarum
    n 1: an Asiatic herb cultivated in Europe for its sweet edible
         tuberous root [syn: skirret, Sium sisarum]