The act of putting saliva on one's penis before masturbating.
This can be in the form of directly spitting upon one's genitalia or somehow placing saliva upon your hand. The saliva can belong to anyone. Those who sloppy are known to scream/shout "Sloppy!!" upon reaching climax and are called 'Sloppyheads', 'Sloppers' and 'Slopdogs' amongst other names.
That was one hell of a sloppy! ! !