Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ex-commūnĭco, āvi, ātum, 1, v. a. (lit., to put out of the community; hence, in eccl. Lat.), to lay under the ban of the Church, to excommunicate, Hier. adv. Ruf. 2, 18: aliquem excommunicatum habere, Hilar. Op. Hist. Fragm. 11, 4.