Noun
(euphemistic) The word illegal, illegal alien, or illegal immigrant.
(humorous) Any word beginning with i that is considered (often humorously) to be taboo in the given context, such as idiot, or incest.
Source: en.wiktionary.orgTo a physicist, we have the 'I' word, the I-word is 'impossible.' That's dangerous. Michio Kaku
Pennebaker crunches the numbers on presidential press conferences since Truman and finds that “Obama has distinguished himself as the I-word user of any of the modern presidents.” Source: Internet