The Collaborative International Dictionary of English v.0.48:

Abirritant \Ab*ir"ri*tant\, n. (Med.)
   A medicine that diminishes or alleviates irritation.
   [1913 Webster + AS]

The Collaborative International Dictionary of English v.0.48:

Abirritant \Ab*ir"ri*tant\, adj. (Med.)
   Diminishing or alleviating irritation --Stedman.
   [AS]