I came upon this word while investigating timorous and temerity. It means reckless or rash.
So, next time your friend walks out into oncoming traffic, pull them back and tell them to stop being so temerarious. They will be so nonplussed, they will heel and walk quietly on the pavement.

Learn a new word every day.
Repeat it and remind yourself what it means at least three times in a day.
Try to use the word in conversation or writing today.
Get a dictionary and look words up.