Irritant

a.	1.	(Scots Law) Rendering null and void; conditionally invalidating. The states elected Harry, Duke of Anjou, for their king, with this clause irritant; that, if he did violate any part of his oath, the people should owe him no allegiance. - Hayward.
	1.	Irritating; producing irritation or inflammation.
n.	1.
	1.	That which irritates or excites.
	2.	(Physiol. & Med.) Any agent by which irritation is produced; as, a chemical irritant; a mechanical or electrical irritant.
	3.	(Toxicology) A poison that produces inflammation. Counter irritant See under Counter. Pure irritant (Toxicology) a poison that produces inflammation without any corrosive action upon the tissues.

Noun

1.

irritant - something that causes irritation and annoyance; "he's a thorn in my flesh" Synonyms: thorn

annoy, annoyance, bother, botheration, chafe, devil, get at, get to, gravel, infliction, irritate, nark, nettle, pain, pain in the ass, pain in the neck, rag, rile, thorn, vex