As a verb, to adulterate means to “corrupt, debase, or make impure by adding a foreign or inferior substance”. The adjective, either adulterated or adulterate (pronounced like the adjective form of moderate), means “mixed with impurities”.
The word unadulterated is much more common, as in the phrase “pure, unadulterated fun.” A Google search for “adulterated” has about 1,260,000 results, while a search for “unadulterated” has 2,300,000. However, the same searches on Google News have about 300 results for both words. I guess those journalists just love to write about “adulterated pet food” and the countless other things that are now being adulterated.
According to the Online Etymology Dictionary, the word comes from Latin “adulterare ‘corrupt, debauch,’ from ad- ‘to’ + L.L. alterare ‘to alter’”. Adultery comes from that same Latin word, but adult actually doesn’t seem to be related.