This section is from the book "Hill's Manual Of Social And Business Forms: A Guide To Correct Writing", by Thos. E. Hill. Also available from Amazon: Hill's Manual Of Social And Business Forms: The How-To-Do-Everything Book Of Victorian America.
Imperious - commanding, domineering, haughty, imperative, proud, lordly, overbearing, tyrannical.
Impertinent - rude, quarrelsome, intrusive, insolent, meddling, irrelevant, troublesome.
Impetuous - hasty, rough, vehement, violent, forcible, boisterous.
Implicate - involve, embarrass, entangle.
Implore - beg, beseech, ask, entreat, supplicate, solicit, request.
Imply - mean, signify, denote, infer, involve.
Importance - weight, moment, signification, consequence.
Imposture - deceit, cheat, fraud, deception, imposition, counterfeit, artifice.
Imprecation - execration, curse, malediction, anathema.
Improve - cultivate, correct, reform, rectify, amend, advance.
Impudent - insolent, bold, rude, saucy, impertinent, uncouth, immodest, shameless.
Impute - charge, ascribe, attribute.
Inability - disability, weakness, impotence.
Inactive - sluggish, lazy, idle, slothful, inert, drowsy.
Inadequate - insufficient, incompetent, unable, incapable.
Inattentive - negligent, heedless, careless, inadvertent, thoughtless, dilatory, remiss.
Incessantly - constantly, continually, unremitingly, unceasingly.
Incident - contingency, circumstance, event.
Incite - provoke, excite, stimulate, arouse, encourage, animate, aggravate.
Include - contain, enclose, comprise, embrace, comprehend.
Incommode - molest, disturb, inconvenience, trouble, annoy.
Incompetent - inapt, insufficient, incapaole, inadequate, unsuitable.
Inconsistent - incongruous, contrary, ridiculous, absurd.
Inconstant - unstable, uncertain, fickle, variable, changeable, versatile.
Indecent - unbecoming, impudent, immodest, indelicate.
Indicate - show, mark, point out, reveal.
Indifferent - passive, neutral, regardless, unconcerned, impartial.
Indigence - poverty, need, want, penury.
Indigenous - native.
Indignation - temper, anger, displeasure, contempt, resentment, wrath.
Indiscretion - imprudence, folly, injudiciousness.
Indispensable - important, necessary, essential.
Indisputable - undeniable, indubitable, unquestionable, incontrovertible, conclusive, settled.
Indistinct - confused, ambiguous, doubtful, dark.
Induce - persuade, lead, influence, urge, instigate, actuate.
Industrious - diligent, persevering, laborious, assiduous, active.
Inevitable - unavoidable, certain.
Inexorable - immovable, relentless, unyielding, implacable.
Inexpedient - unsuitable, unfit, inconvenient.
Infect - taint, corrupt, defile, contaminate, pollute. Inference - deduction, conclusion.
Inferior - less, lower, secondary, subservient, subordinate.
Infested - disturbed, troubled, annoyed, plagued.
Infinite - boundless, unbounded, illimitable, unlimited, immense, eternal.
Infirm - weak, sickly, decrepit, feeble, debilitated, imbecile.
Influence - authority, power, persuasion, credit, favor, sway.
Information - notice, counsel, intelligence, advice, instruction.
Ingenious - inventive, talented, skillful.
Ingenuity - capacity, invention, genius, skill, talent.
Inhabit - dwell, occupy, reside, stay, abide, sojourn.
Inherent - innate, inborn, inbred.
Inhuman - cruel, savage, barbarous, brutal.
Iniquitous - unjust, evil, wicked, nefarious.
Injunction - order, mandate, precept, command.
Injure - harm, hurt, impair, damage, deteriorate.
Innate - natural, inherent, inbred, inborn.
Innocent - pure, blameless, guiltless, faultless, inoffensive, harmless, spotless.
Inordinate - immoderate, intemperate, irregular, excessive.
Inquisitive - curious, inquiring, anxious, prying.
Insanity - derangement, madness, craziness, lunacy, mania.
Insensibility - dullness, apathy, indifference, stupidity, torpor, imperceptibility.
Insidious - deceitful, sly, crafty, cunning, subtle treacherous.
Insignificant - worthless, meaningless, inconsiderable, trivial, unimportant.
Insinuate - hint, suggest, intimate.
Insolent - insulting, abusive, rude, haughty, saucy, offensive, impertinent.
Inspire - animate, invigorate, enliven, cheer, exhilarate, suggest.
Instigate - tempt, incite, urge, encourage, impel, move, stimulate.
Instill - infuse, implant, sow.
Insufficient - inadequate, incapable, incompetent, unfit, unable, unsuitable.
Insult - abuse, affront, outrage, contempt, insolence, indignity.
Integrity - purity, probity, truthfulness, uprightness, honesty.
Intellect - understanding, genius, ability, capacity, talent.
Intelligence - intimation, understanding, information, notice, knowledge, intellect.
Intemperate - excessive, immoderate, inordinate.
Intend - purpose, mean, design.
Intercede - mediate, interpose, interfere.
Interline - insert, alter, correct, add.
Intermission - cessation, stop, rest, vacation, interruption.
Intermit - abate, suspend, subside, forbear.
Interpose - mediate, interfere, intermeddle.
Interpret - explain, demonstrate, elucidate, expound, decipher.
Interrogate - examine, question, inquire.
Interval - space, interstice, time.
Intervening - coming between, interposing, intermediate.
Intimidate - frighten, alarm, daunt, scare.
Intoxication - infatuation, inebriety, drunkenness.
Intractable - perverse, obstinate, stubborn, ungovernable, uncontrollable, unmanageable.
Intrepid - fearless, undaunted, bold, daring, valiant, courageous, brave.
Intrinsic - real, true, inherent, inward, essential, genuine.
Introductory - preliminary, previous, prefatory.
Intrude - invade, infringe, encroach, obtrude, entrench. Intrust - confide, commit.
Invade - enter, attack, intrude, encroach, infringe. Invalid - weak, sick, infirm, null, feeble, void.