For many artists, painting serves as a primary outlet for their innermost thoughts.

outlet 由 out（向外）和 let（让、允许）组成，字面意思是"让...出去的地方"。引申为"发泄/表达（情感、想法等）的途径或方式"，在这里指艺术家通过绘画来表达内心想法的渠道。