1. To fix, fasten, or secure something onto something or some surface with or as with tacks or small nails. I just need to tack some aluminum sheeting onto the side of the barn. He's been going around tacking his posters onto the walls around campus.

2. To add something onto something else, especially that which is unwanted or superfluous. It feels like the filmmakers tacked the love story onto the plot at the last minute. They tacked a number of service charges onto my bill that they were not able to explain. If you kids keep talking, I'll tack an extra 50 pages onto your homework for the weekend.