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An international version of the Scandinavian "Fultal", literally: "Ugly Number". Simply the amount of partners a person has slept with, multiplied by the difference in age between the youngest and oldest.

Ex. 15 partners, youngest born 1985, oldest 1975, filth factor of 150.
- Karen spent the night at that 65 year old's place.

- Oh my god, her filth factor must have gone through the roof!!
by Carl Philip March 11, 2008
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