If you conk someone, you bang them on the head. You can also conk your own head on a low-hanging light fixture or cabinet door.
Conk is an informal way to say "hit" or "bang," especially when it's someone's head bumping into something. You can also conk out, or collapse from exhaustion — and your motorcycle can also conk out, when it breaks down. Originally, conk meant "punch in the nose," from 19th century slang for "nose," conk. Later, the word became World War I military slang, for the sound a stalling plane engine makes when it "conks out."