When something stultifies you, it drains you of your energy, enthusiasm, or pleasure. Listening to an hour-long graduation speech on a hot, humid day might stultify you.
The word stultify is sometimes used in place of to "bore" or "exhaust.” An uninspiring boss, too many rules, or a badly performed play can be stultifying. If you hang out in the hot sun all day, the stultifying heat might make you feel limp and listless. Even constant construction noise could stultify a person. To stultify a plan or person is to dampen enthusiasm in a big way.