complete lie; outright lie
Emphatic expression for a blatant, obvious lie. The image is of a lie so shameless it is 'bright red'.
His excuse was a complete lie.
That's obviously an outright lie.
Standard kanji spelling; 真っ赤 (bright red) + 嘘 (lie).
Katakana spelling of 嘘 is common in casual writing.
From 真っ赤 (bright red) + 嘘 (lie). The red color metaphorically represents something blatant or shameless, similar to English 'barefaced lie'.