Understanding 'Irritable' in Context

As a consequence of his incessant questioning, the students became increasingly irritable during the lecture. The professor, known for his engaging style, found it challenging to maintain the original enthusiasm of the class. Despite his efforts to redirect the discussion, the atmosphere grew tense, and it became evident that his usual rapport with the students was fraying at the edges.

In this context, the word 'irritable' most closely means: