noun
makeup base; foundation
Refers to the base layer applied before makeup, such as primer or foundation. Often used in cosmetics contexts.
化粧下を塗ると、ファンデーションのノリが良くなる。
Applying a makeup base helps the foundation go on more smoothly.
Compound of 化粧 (makeup) and 下 (under, base). The reading した is the kun'yomi of 下, forming a straightforward noun meaning 'under-makeup' or 'makeup base'.