Boracic lint was a type of medical dressing made from surgical lint that was soaked in a hot, saturated solution of boracic acid

Boric acid

Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...

 and glycerine

Glycerol

Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...

 and then left to dry.

It has been in use since at least the 19th century, but is now less commonly used.

The term boracic lint, or often just "boracic", pronounced "brassic", is also used as Cockney rhyming slang

Cockney rhyming slang

Rhyming slang is a form of phrase construction in the English language and is especially prevalent in dialectal British English from the East End of London; hence the alternative name, Cockney rhyming slang...

for having no money. Boracic lint → skint.