Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
obsĭdātus, ūs, m. [obses], the condition of a hostage, hostageship (post-class.): pater ejus diu obsidatūs pignore tentus, Amm. 16, 12, 25: obsidatūs sorte in Syriis detentus, id. 18, 6, 20: in obsidatum datus, Schol. Juv. 2, 164.