The Collaborative International Dictionary of English v.0.48:
Tamper \Tam"per\, v. i. [imp. & p. p. Tampered; p. pr. & vb.
n. Tampering.] [A corruption of temper.]
1. To meddle; to be busy; to try little experiments; as, to
tamper with a disease.
'T is dangerous tampering with a muse. --Roscommon.
2. To meddle so as to alter, injure, or vitiate a thing.
3. To deal unfairly; to practice secretly; to use bribery.
For Fleetwood, Desborough, and Lambert. --Hudibras.