incomprehensible - Impossible or extremely difficult to understand
infatuation - An intense but short-lived love, attraction, or admiration for someone or something.
inferior - Low in rank, status, imprtance, or quality.
infinitesimal - Extremely tiny, almost too small to be measured.
infamous - Well known for some bad quality or deed
impregnable - Unable to be broken into or captured.
impediment - Something that makes progress or movement difficult or impossible
infatuated - Having an intense but often short-lived passion or admiration for someone.
impractical - Not useful, unlikelly to succeed, not sensible or realistic.
indiscriminate - Not showing careful choice or planning, often resulting in harmful results
indifferent - Not showing or feeling interest or concern about something.
inconceivable - Not capable of being imagined or grasped mentally; unbelievable