To contemplate, ruminate over, or reflect on something. I've been thinking on this issue all week, and I've come up with a few possible solutions. A: "I'm not willing to uproot my whole family just for some job." B: "Look, just think on it for a couple days, all right?" I found myself thinking on my best friend from elementary school the other day. I wonder what ever happened to him?

to contemplate someone or something; to muse or reflect on someone or something. (Upon is formal and less commonly used than on.) I thought upon Abraham Lincoln and how much we all owe him. I thought on all the fine things we used to do.