knob vs letterbox

knob

noun
  • A rounded protuberance, especially one arising from a flat surface; a fleshy lump or caruncle. 

  • A ball-shaped part of a handle, lever, etc., designed to be grabbed by the hand. 

  • The head of the penis; the glans. 

  • A woman's breast. 

  • A contemptible person; dick. 

  • A prominent rounded hill. 

  • A freshman at The Citadel, The Military College of South Carolina. 

  • The clitoris. 

  • A rounded control switch that can be turned on its axis, designed to be operated by the fingers. 

  • A prominent, rounded bump along a mountain ridge. 

  • A dollop, an amount just larger than a spoonful (usually referring to butter). 

  • A chunky branch-like piece, especially of a ginger rhizome. 

  • A bulb of the garlic plant consisting of multiple cloves. 

  • A rounded ornament on the hilt of an edged weapon; a pommel. 

verb
  • To have sex with. 

letterbox

noun
  • A rectangular hole or slot. 

  • A hidden container that holds a logbook and rubber stamp, found by following clues as a form of recreation. 

  • A delivery point for mail, a box, compartment or slot for this purpose. 

  • A collection point for mail intended for onward delivery, a secure box or receptacle for this purpose. 

verb
  • To transfer a widescreen motion picture to home video formats while preserving the original aspect ratio, with the placing of black bars above and below the picture area. 

  • To hunt for letterboxes (containers with logbook and rubber stamp) by following clues. 

adj
  • Transferred to home video formats while preserving the original aspect ratio, having black bars above and below the picture area. 

How often have the words knob and letterbox occurred in a corpus of books? (source: Google Ngram Viewer )