Jacksy means: British slang for the backside or buttocks - one of those affectionate, slightly childish-sounding words that adults use to soften what they're actually talking about. Jacksy has a comic quality to. It is informal slang, fine in casual speech but not suited to formal contexts. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. This is a food-related word and harmless in itself.