"The ancestral meaning of thing is time: it goes back to a prehistoric Germanic *thingam, which was related to Gothic theihs time, and may come ultimately from the Indo-European base *ten stretch (source of English tend, tense, etc). In Germanic it evolved semantically via appointed time to judicial or legislative assembly. This was the meaning it originally had in English, and it survives in other Germanic languages (the Icelandic parliament is known as the Althing, literally general assembly). In English, however, it moved on through subject for discussion at such an assembly to subject in general, affair, matter and finally entity, object. (The ancient meaning assembly is preserved in fossilized form in English husting, etymologically a house assembly)."
~ John Ayto, "Dictionary of Word Origins."