- invert: turn upside down or inside out
- irascible: irritable, easily angered
- ironic: humorously sarcastic or mocking
- irrational: illogical, lacking reason, insane
- irrelevant: not applicable, unrelated
- irreproachable: blameless, impeccable
- irresolute: uncertain of how to act, weak
- irreverence: showing a lack of respect
- jargon: language specific to a certain group of people (usually job related)
- jocular: joking