Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
‡ ŏb-ultrōnĕus, a, um, adj., voluntary, spontaneous: ultroneus, obultroneus, Not. Tir. p. 83: ‡ obultroneus, αὐθαίρετος, Gloss. Philox.