When the act of maturbation is disturbed causing loss of momentum or ejaculation in your pants because some inconsiderate person has turned up uninvited

The realisation that your child is a fuckwit and you would rather of just had a wank that night.

Sperm that was ejaculated due to masturbation and left to waste.

The sperm from masturbation that would be discarded in a tissue and thrown away.

To call someone a 'wank waste' would be to insinuate that they are unworthy sperm or a worthless human undeserving of the life they have.