thumbtack

A tack with a smooth, rounded head that can be pressed into place with the thumb.

To affix with a thumbtack.

A small nail-like tack with a slightly rounded head that can be pressed into place with light pressure from the thumb; used for hanging light articles on a wall or noticeboard

To fix or attach something with a thumbtack.

fasten with thumbtacks

a tack for attaching papers to a bulletin board or drawing board