late Middle English compoune (verb), from Old French compoun-, present tense stem of compondre, from Latin componere ‘put together’. The final -d was added in the 16th century on the pattern of expound and propound. compound arose through a misinterpretation of the legal phrase compound a felony, which means ‘refrain from prosecuting a felony in exchange for money or other consideration’. This led to the use of compound in legal contexts to mean ‘make something bad worse’, which then became accepted in general usage.