Is it farther or further?
early 15c., in reference to sores and morbid swelling, from Middle French irritation or directly from Latin irritationem (nominative irritatio) "incitement, irritation," noun of action from past participle stem of irritare (see irritate).
irritation ir·ri·ta·tion (ĭr'ĭ-tā'shən)
Extreme incipient inflammatory reaction of the body tissues to an injury.
The normal response of a nerve or muscle to a stimulus.
The evocation of a reaction in the body tissues by the application of a stimulus.