expression, noun
complete lie; outright lie
Emphatic expression for a blatant, obvious lie. The image is of a lie so shameless it is 'bright red'.
彼の言い訳は真っ赤な嘘だった。
His excuse was a complete lie.
そんなの真っ赤な嘘に決まってる。
That's obviously an outright lie.
From 真っ赤 (bright red) + 嘘 (lie). The red color metaphorically represents something blatant or shameless, similar to English 'barefaced lie'.