Wiktionary
a. 1 Having a very thin neck. 2 (context idiomatic US pejorative English) Insubstantial; weak.
Usage examples of "pencil-necked".
In the dream it's the figure of a very tall sunken-chested man in black-frame glasses and a sweatshirt with old stained chinos, leaning back sort of casually or else morosely slumped, resting its tailbone against the window sill's ventilator's whispering grille, with its long arms hanging at its sides and its ankles casually crossed so that Gately can even see the detail that the ghostly chinos aren't long enough for its height, they're the kind kids used to call 'Highwa-ters' in Gately's childhood a couple of Bimmy Gately's savager pals would corner some pencil-necked kid in those-type too-short trousers on the playground and go like 'Yo little brother where's the fucking flood?
Shaftoe and the other Marines have always known Station Alpha as a mysterious claque of pencil-necked swabbies who hung out on the roof of a building in the International Settlement in a shack of knot-pocked cargo pallet planks with antennas sticking out of it every which way.