to meddle, esp. for the purpose of altering, damaging, or misusing (usually fol. by with): Someone has been tampering with the lock.
to make changes in something, esp. in order to falsify (usually fol. by with): to tamper with official records.
to engage secretly or improperly in something.
to engage in underhand or corrupt dealings, esp. in order to influence improperly (usually fol. by with): Any lawyer who tries to tamper with a jury should be disbarred.