Merriam-Webster Collegiate Dictionary - tamper

 intransitive verb  (~ed; ~ing)  Etymology: probably from Middle French temprer to temper, mix, meddle — more at temper  Date: 1567  1. to carry on underhand or improper negotiations (as by bribery)  2.  a. to interfere so as to weaken or change for the worse — used with with did not want to ~ with tradition  b. to try foolish or dangerous experiments — used with with  c. to render something harmful or dangerous by altering its structure or composition was charged with ~ing with consumer products  • ~er noun  • ~proof adjective