Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Sescentō-plāgus, i, m. [sescentiplaga], a man of six hundred stripes, a name coined by Plautus: nisi cottidiano sesquiopus confeceris, Sescentoplago nomen indetur tibi, Plaut. Capt. 3, 5, 68.