A pinhead is the tiny flat part at the top of a straight pin, the bit you push with your finger when sticking the pin through fabric. It's remarkably small, which is why people use the phrase “the size of a pinhead” to describe something extremely tiny. You might read that a rare insect is “no bigger than a pinhead” or that someone wrote their name in letters “the size of a pinhead.”

The word also gets used as an insult, though it's not a particularly harsh one. Calling someone a pinhead means you think they're being foolish or not thinking clearly. If your brother insists that Antarctica is the hottest continent, you might mutter, “Don't be such a pinhead!” It's the kind of insult that suggests someone is acting silly or stubborn rather than genuinely unintelligent. The image is of someone whose brain is as small as the head of a pin.

You'll sometimes see pinhead used more literally in old stories or historical descriptions, where it simply refers to that actual tiny piece of metal atop a pin, without any insulting meaning at all.