Answer:
lack of enthusiasm
Step-by-step explanation:
In this passage, we see Brutus talking about Octavius' army. He tells us that he senses that Octavius' wing has a cold demeanour. This "coldness" is referred to in order to contrast it with the "heat" of battle. Brutus implies that he thinks Octavius' army is not particularly enthusiastic. Therefore, he urges Messala to let Cassius know of this, as he wants them to exploit such weakness.