Hydrocyanic acid is a weak acid because it is not completely ionized when it is dissolved in water. Strong acids can be ionized completely in water.
Strong and weak acids are mainly distinguished by how likely the acid is to dissociate into its constituent ions when it is dissolved in water. Strong acids tend to ionize completely and split into a hydrogen ion (which combines with the water molecule) and an anion. With weak acids such as hydrocyanic acid, the acid does not completely ionize. Instead, a chemical equilibrium exists between the ionized and unionized forms of the acid in a reversible reaction.