Underkecks means: A delightfully blunt Northern English and Scottish term for underpants. 'Kecks' is already regional slang for trousers, so 'underkecks' naturally refers to what's worn underneath - your underwear,. It is mainly Scottish English and may not be understood outside that context. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. As a place or proper-name reference it is purely descriptive.