tribo-, trib- +

To produce light by friction or light that has been produced with friction.

An instrument for examining triboluminescence [glow or emission of light that results from friction or mechanical pressure].

The physical properties or phenomena associated with friction.

Luminescence resulting from exposure to high temperature, produced in a material as a result of friction.

Thermoluminescence (luminescence resulting from exposure to high temperature) produced in a material as a result of friction.

1. Something that causes great difficulty, affliction, or distress; such as, an ordeal: "I have experienced the trials and tribulations of a struggling lexicographer."
2. An experience that tests one's endurance, patience, or religious faith.
3. Etymology: from Old French tribulacion (12th century), from Late Latin tribulationem, tribulatio, "distress, trouble, affliction"; from tribulatus, tribulare, "to oppress, afflict"; a figurative use by Christian writers of Latin tribulare "to press"; also possibly, "to thresh out grain;" from tribulum "threshing sledge", from stem of terere, "to rub" + -bulum, a suffix forming names of tools.

It originally came from Greek; then through Latin, "to press; affliction"; and by extension, "distress, great trial", or "affliction".

The Roman tribulum was a sledge consisting of a wooden block studded with sharp pieces of flint or iron teeth. It was used to bring force and pressure against wheat in grinding out grain.

The machine suggested the way trouble grinds people down and oppresses them, tribulations becoming another word for troubles and afflictions. The word is first recorded in English in 1330.

The Romans ground out their corn (grain) with a heavy roller, mentioned in Vergil’s Georgics among agricultural instruments: the tribulum, a diminutive noun, from tritere, trit-, "to rub", from Greek tribein, "to rub".

"Being ground under and pressed out" made an excellent metaphor to express the trials and tribulations of the early Christians.