UN
impossible to think
unthinkable
doing less than 100%
inefficient
not connected to the main topic
irrelevant
contrary to logic
illogical
about someone who doesn't follow a social code of rules (may also sound religious)
immoral
not mainstream, not liked by the majority
unpopular
about someone who doesn't care for other people's feelings
inconsiderate
impossible to find another with the same function
irreplaceable
not possible to read
illegible
improper
looking very different, not resembling oneself
unrecognizable
about something which you cannot see (sometimes because of magic)
invisible
so strong that it has to be followed, e.g. a temptation
irresistible
not recognized by law, for example about a child out of wedlock
illegitimate
immortal
not possible to predict, unexpected future development
unforeseeable
about something which seems harmless although many people see it the other way, e.g. a joke
inoffensive
beyond fixing
irreparable
unable to write or read
illiterate
not moving at all
immobile