What is the sin of presumption?
The short answer
Presumption is relying on God's mercy and power in a disordered way — hoping for salvation without doing anything for it, or for pardon without repenting. Deciding to sin because one can be forgiven is to sin twice: it uses God and makes light of his Passion.
Definition
The Catholic Encyclopedia defines presumption as a disordered reliance on God's mercy and power that hopes for salvation without doing anything to deserve it, or for pardon without repenting.
Sinning twice
How sincere is someone's love for God if he is willing to sin because God forgives? God reads hearts. To decide to sin offends God; to decide to sin because one can be forgiven is to sin twice — using God as a convenience and mocking his passion and death.
A prayer
Adapt the Divine Mercy Chaplet: offer the Father the body and blood, soul and divinity of his Son, in atonement for our sins and for protection against this temptation, and ask mercy for the sake of his sorrowful passion.