He always talks about abstract paintings even though he knows nothing about art—it seems so pretentious.

pretentious 是由名词 pretension（自命不凡）加上形容词后缀 -ious 构成的形容词，意思是"自命不凡的、做作的"，形容某人刻意表现出比自己实际水平更高的样子。