Angrier means more filled with anger than before, or than somebody else. The temperature has gone up.
It is the comparative of angry, and it follows the ordinary rule for short adjectives ending in a consonant and y, which change to i before -er. Angry, angrier, angriest. It takes than for the comparison, and about or with for the cause. There is a common construction in getting angrier and angrier, which describes a slow build rather than an explosion, and it is one of the standard ways English shows a rising temper.
1adjectiveangrierMore angry than before or than someone else.
“The longer he waited the angrier he became.”
Middle English
angry
English
angry
English, today
angrier
angry comes from Middle English angry. From Middle English angry, equivalent to anger + -y; see anger.
Etymology from Wiktionary, CC BY-SA