thick [OE] The ‘slow-witted’ sense of this Germanic word dates from the late 16th century. In Shakespeare’s Henry IV, Part 2 Falstaff says disparagingly of his companion Poins: ‘His wit’s as thick as Tewkesbury mustard’. A very stupid person might be as thick as two short planks, thick as a plank or thick as a brick—there is a play on thick in the usual sense ‘deep from side to side’ and the sense ‘stupid’. Thick [M18th] with the meaning ‘very friendly’, as in thick as thieves, comes from the sense ‘very close together, tightly packed’. To go through thick and thin goes back to medieval times and originally probably referred to someone pushing their way both through a thicket (a related OE word), where trees grew closely, and ‘thin wood’, where the going would be easier. See also callous.