Final answer:
The chairman's mistake of saying "pineapple" instead of "pinnacle" is a malapropism, a slip of the tongue where a word is mistakenly used in place of another that sounds similar, creating an unintentional error or humor.
Step-by-step explanation:
When the chairman in a board meeting mistakenly says "John is the pineapple of hard work" instead of "pinnacle", he has made a slip of the tongue known as a malapropism. A malapropism occurs when an incorrect word is used in place of a word with a similar sound, resulting in a nonsensical or humorous expression. In the case provided, "pineapple" does not fit the intended meaning of being the highest point or peak of hard work, which is what "pinnacle" refers to. This mistake is different from a synecdoche, which is a figurative language where a part is used to represent the whole or vice versa, such as saying "he relinquished the crown" when indicating that a king has given up power, where the crown is symbolic of the entire role of the king.