Thinner (noun)

1. A liquid that is used to dilute or reduce the consistency of paints, varnishes, or other substances.
2. A solvent that is used to dissolve or reduce the viscosity of other substances.

Origin:

From middle english thinne, from old english thin, thinne, from proto-germanic *thinnaz.

Examples:

1. He used a paint thinner to clean the brushes.
2. The nail polish was too thick, so I added some thinner to it.
3. The glue was too sticky, so I used a thinner to soften it.
4. The artist was careful to measure the right amount of thinner for the paint.
5. She went to the hardware store to buy some thinner for her varnish.