To tell a big lie or to boast/exaggerate; to talk nonsense or brag (idiomatic, colloquial).
彼は自分の武勇伝を誇張して大ぼらを吹いた。
He exaggerated his exploits and told a tall tale.
あまり大法螺を吹くと信用を失うよ。
If you keep blowing big lies, you'll lose credibility.
あのセールスマンはよく大ぼらを吹くので、話半分に聞いた方がいい。
That salesman often tells tall tales, so you should take what he says with a grain of salt.