The phrase "one corner short of a pentagon" is a humorous idiom used to describe someone perceived as lacking intelligence or common sense. It fits within a broader category of expressions that convey a similar meaning, such as "one card short of a full deck" or "one brick short of a full load." Participants in the discussion note that these sayings often use geometric or numerical imagery to illustrate the idea of incompleteness in someone's mental capacity. The conversation also explores variations and creative adaptations of these phrases, highlighting the playful nature of language. Additionally, the term "square" is discussed, indicating that in American vernacular, it can imply someone who is conventional or out of touch rather than directly referencing intelligence. Overall, the thread emphasizes the humor and creativity inherent in these expressions while clarifying their meanings.