Rashness is the quality of doing things too quickly, without thinking carefully whether they are sensible. We talk about the rashness of a plan, or someone's rashness in promising too much. It is a slightly formal noun; in everyday talk people more often say someone was too hasty, or they didn't think. You will meet it in essays, news, and gentle warnings. Though actually, a little boldness can look like rashness to a very careful person, so the word can be a bit unfair if you only see the risk and not the reason. It names a way of deciding, not a skin problem.