To wank, or be wanked, in a particularly aggressive or over-enthusiastic manner. Usually involves an element of discomfort on the part of the person whose penis is being so manipulated.
Origins disputed, though commonly thought to
stem from the mastubatory practices of Slovenian
men, who, too used to driving tractors with rusty or stiff
gear sticks, would manipulate their penises in a similarly vigorous fashion.
Nowadays if you are being wanked in such a fashion it is unlikely that
the man wanking you is a genuine Slovenian tractor driver.