In medieval Europe what did the word "thing" mean?

A thing (that is, "assembly" or folkmoot) was a governing assembly in early Germanic society, made up of the free people of the community presided over by a lawspeaker. Things took place at regular intervals, usually at prominent places that were accessible by travel. They provided legislative functions, as well as being social events and opportunities for trade. In modern usage, the meaning of this word in English and other languages has shifted to mean not just an assemblage of some sort but simply an object of any sort.

The Old Germanic form of the word ‘thing’ is þingsō which derives from the word þengaz, and which means ‘certain time’. In Gothic it is þeihs meaning time. It was therefore a specific time the people gathered, and that is how the word thing received the meaning of folkmoot and assembly, and of justice.

The Old Norse, Old Frisian, and Old English þing with the meaning "assembly" is identical in origin to the English word thing, German Ding, Dutch ding, and modern Scandinavian ting when meaning "object". All of these terms derive from Proto-Germanic þingą meaning "appointed time", possibly originating in Proto-Indo-European *ten-, "stretch", as in a "stretch of time for an assembly".