The phrase "old toothbrush" refers to a toothbrush that is no longer new or recently bought. It emphasizes that the toothbrush has been used for a while and might not work as effectively as a new one. Full definition
So the next time you're looking for insulation in your local DIY store, just think in a few years» time you could be browsing other peoples old toothbrushes instead! (idealhome.co.uk)
Let it absorb into the fabric, then brush the stain gently with an old toothbrush. (sense-organics.com)