Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. dēcoctus, a, um, Part., from decoquo.
* 2. dēcoctus, ūs, m. [decoquo], a boiling down, seething: gemmae mellis decoctu nitescunt, Plin. 37, 12, 74, § 195.