what did the people in the country of origin use Teff for in the past?

it is a grain and it was used for food.

Eragrostis tef, known as teff, taf (Amharic ጤፍ ṭēff, Tigrinya ጣፍ ṭaff), is an annual grass, a species of lovegrass native to the northern Ethiopian Highlands of Northeast Africa...

Eragrostis tef has an attractive nutrition profile, being high in dietary fiber and iron and providing protein and calcium.[2] Some consider it to have a sour taste. This might be because it is often eaten fermented. It is similar to millet and quinoa in cooking, but the seed is much smaller and cooks faster, thus using less fuel...

Teff is believed to have originated in Ethiopia between 4000 BCE and 1000 BCE.