A loggerhead is a large sea turtle with a massive, block-shaped head and powerful jaws. These turtles can weigh over 250 pounds and spend most of their lives swimming thousands of miles across oceans, coming ashore only when females need to lay eggs on sandy beaches.

The word also appears in the phrase at loggerheads, which means locked in a serious disagreement or argument. When two people are at loggerheads, they strongly oppose each other and can't find common ground. Two friends might be at loggerheads over which game to play, or scientists might be at loggerheads about how to interpret their data.

The turtle's solid, stubborn-looking head can suggest the image of two opponents facing off, neither willing to budge. When people are at loggerheads, they're stuck: both sides believe they're right, and neither wants to compromise. The expression captures that feeling of being locked in conflict and unable to move forward.