Walnut ink is an ink

Ink

Ink is a liquid or paste that contains pigments and/or dyes and is used to color a surface to produce an image, text, or design. Ink is used for drawing and/or writing with a pen, brush, or quill...

 made from the green husk surrounding the nut

Nut (fruit)

A nut is a hard-shelled fruit of some plants having an indehiscent seed. While a wide variety of dried seeds and fruits are called nuts in English, only a certain number of them are considered by biologists to be true nuts...

 of walnut

Walnut

Juglans is a plant genus of the family Juglandaceae, the seeds of which are known as walnuts. They are deciduous trees, 10–40 meters tall , with pinnate leaves 200–900 millimetres long , with 5–25 leaflets; the shoots have chambered pith, a character shared with the wingnuts , but not the hickories...

s. It may be liquid or made of crystals that are mixed with water before use. It can be used to produce stains and darken paper to make it look older.

An ancient use of Walnut ink was to stain the hands of criminals in gypsy communities. Once stained, it is impossible to wash off and remains in the skin for long period of time.