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Under the spreading chestnut tree
The village blacksmith sat
By abusing himself
And coming off in his hat
- Henry Wadsworth Longfellow
From the day the male foetus' hands grow long enough to grasp at their 'third leg', until the man in question is dead and buried, the penis is a constant source of amusement and amazement to those of the male gender.
At some point in their lives, all men experience periods where sexual companionship is lacking. They must therefore resort to the stimulation or manipulation of their own genitals in order to achieve orgasm. Couples often sub-contract masturbation to each other, although masturbating somebody else requires polished technique. For the sake of a relationship, the recipient will often report the experience pleasurable, before running off to the loo to cover his penis area with soothing cream.
The English language offers a plethora of different words and terms to describe this act. A few examples are: tossing, ham-boning, beating the dummy/meat and spanking the monkey.
In times past, sexual self-gratification was considered an unclean act, even a sin. Sperm was believed to have been created for procreational purposes only. Nowadays masturbation is not a punishable offence in most civilised countries, although tossing off in public is generally frowned upon.
Implying somebody masturbates is considered an insult although it's the only insult that's true of 99% of Earth's population. The remaining 1% are very tense.