Describing someone who is simple-minded, not very intelligent. The top floor in this slangy insult denotes the brain. One synonym is a few/two/three bricks shy of a load, indicating a person is short of intelligence. Another is not playing with a full deck, which refers to the card game of poker. Yet another is having only one oar in the water (or not having both oars in the water). All these slangy expressions date from the second half of the twentieth century. For example, “But now this new opportunity had presented itself, and . . . how could he really lose? Okay, she probably wasn’t playing with a full deck, but he didn’t figure her for any more gun wielding” (David Baldacci, Hour Game, 2004).