To tackle a difficult problem boldly.

This little figure of speech is known wherever Urtica Dioica, the Stinging Nettle, is commonplace, which covers most of the English-speaking world. The figurative advice to be bold and 'grasp the nettle' derives from the property of the plant to inject toxins into the skin of any person or animal who brushes against its stiff, hollow hairs. If the plant is grasped firmly, especially if that is done in the direction the hairs are growing, the hairs tend to be pushed flat and avoid penetrating the skin.

Nettles favour disturbed ground and consequently are often found near human habitation. Fortunately, the antidote to nettle stings is found in the leaves of Dock, which also grows on disturbed soil and is usually to be found near nettles.

The property of the plant was well enough known by the 16th century for John Lyly to have included a reference to it in Euphues, 1578:

"True it is Philautus that he which toucheth ye nettle tenderly, is soonest stoung."

Aaron Hill's Works, circa 1750, contains the first example that I can find that advises that a nettle be grasped:

"Tender-handed stroke a nettle, And it stings you, for your pains: Grasp it like a man of mettle, And it soft as silk remains."