The World Health Organization report in Concise International Chemical Assessment Document 61:
HYDROGEN CYANIDE AND CYANIDES: HUMAN HEALTH ASPECTS
Following application of cyanides in aqueous solution to the intact skin of New Zealand rabbits, the dermal LD50s of hydrogen cyanide, sodium cyanide, and potassium cyanide were 0.260, 0.298, and 0.343 mmol/kg body weight, respectively (corresponding to 6.8, 7.7, and 8.9 mg cyanide/kg body weight) (Ballantyne, 1983a). The dermal toxicity of cyanide, especially of hydrogen cyanide, is markedly greater following application on abraded skin, which enhances the penetration of cyanide (LD50s of 0.087, 0.220, and 0.30 mmol/kg for hydrogen cyanide, sodium cyanide, and potassium cyanide, respectively) (Ballantyne, 1987).5
So, if we assume humans are just like 80kg rabbits, that means that unbroken skin would require 540mg of Hydrogen cyanide or 616 mg of Sodium cyanide.
Hydrogen cyanide is a gas at 26°C, [Wikipedia] so it might be hard to deal with. Let's go with Sodium cyanide, which is a solid that dissolves well into water. At 26°C, Sodium cyanide can be dissolved into water at 52g/100Ml. [Wikipedia], so that corresponds to 1-2 mL of concentrated solution - less than half a teaspoon, for a 50% chance of death.
If you scratch up your victim first, you could reduce it to a 50th of that.
Of course, for governments, Hydrogren Cyanide has plenty of evidence of being a grisly and effective poison: Germany, United States
[Hat-tip to @Fabian who shot down my earlier answer.]