1. To assume a sitting position on top of someone or something. There were no chairs left, so I had to sit down on the coffee table during the party. The woman said she was feeling dizzy, so I told her to sit down on the curb for a minute.

2. To place or set someone or something down on top of someone or something else. In this usage, a noun or pronoun is used between "sit" and "down." The father set the child down on chair in the corner of the room for a time-out. The coach came over and sat a big sandbag down on me to make it harder to do pushups.