乱(luàn) - messy, chaotic, in disorder; recklessly in Mandarin

乱 (luàn) covers every kind of disorder: an untidy room, a chaotic street, a country at war, a head full of worries. 我心里很乱 is the standard way to say you cannot think straight.

The second life of 乱 is the one learners miss. Put it in front of a verb and it becomes an adverb meaning any old how, without thinking, and usually with a note of telling off. 乱说 is to talk rubbish, 乱花钱 is to throw money about, 乱走 is to wander off where you should not. 别乱 plus a verb is one of the most common things a parent says.

The trap is treating it as a simple adjective. Because it already means messy, 乱 needs no verb of its own after 很, and to say something got messy you normally use a result complement instead: 弄乱, 搞乱, 变乱.