Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sur-rūmus (subr-), a, um, adj. [rumis], under the udder, sucking, suckling: agni, Varr. 2, 1, 20; 2, 11, 5; cf. Fest. p. 270 Müll. et v. rumis.