INVINCIBLE—unconquerable, impregnable, insurmountable.
INVISIBLE—unseen, imperceptible, impalpable.
INVITE—ask, call, bid, request, allure, attract.
INVOKE—invocate, call upon, appeal, refer, implore, beseech.
INVOLVE—implicate, entangle, compromise.
IRKSOME—wearisome, tiresome, tedious, annoying. (Pleasant.)
IRONY—sarcasm, satire, ridicule, raillery.
IRRATIONAL—foolish, silly, imbecile, brutish, absurd, ridiculous. (Rational.)
IRREGULAR—eccentric, anomalous, inordinate, intemperate. (Regular.)
IRRELIGIOUS—profane, godless, impious, sacrilegious, desecrating.