Thorn
Part of speech: noun
Definition: A sharp protective spine of a plant. A letter of the Latin alphabet (capital: Þ, small: þ), borrowed by Old English from the futhark to represent a dental fricative, then not distinguished from eth, but in modern use (in Icelandic and other languages, but no longer in English) used only for the voiceless dental fricative found in English thigh
Example sentence: To smile at the jest which plants a thorn in another's breast is to become a principal in the mischief.
Irritant
Part of speech: adjective
Definition: causing irritation or inflammation
Part of speech: noun
Definition: a source of irritation