full of lies; nothing but lies
Emphatic colloquial expression meaning that something is completely untrue or that someone is telling nothing but lies. Often used in informal speech.
What he says is full of lies.
That rumor was nothing but lies.
嘘 (lie) + 八百 (eight hundred, here used as an intensifier meaning 'a great many'). The exact origin of using 八百 for emphasis is uncertain, but it appears in other expressions like 八百屋 (greengrocer, originally 'shop with many goods').