莽 is a bound form (b.f.) meaning 'crude, impetuous, or rude' — it almost always appears as part of compound words like 鲁莽 (lǔmǎng, 'rash, reckless') or 莽撞 (mǎngzhuàng, 'blundering'). Its original sense of 'dense grass' survives in literary compounds like 莽莽 (vast, luxuriant) but is rare in everyday speech. You won't use 莽 alone as a standalone word; learn it inside the compounds where it carries the 'impetuous' meaning.