Simple

Not very intelligent; a person who has difficulty learning things ;lacking in knowledge or expertise

Similar to the word 'daft' which pertains to 'a stupid person; foolish'

Commonly spoken by British English (BrE) speakers, who have quite an extensive vocabulary to express their words.

*friend slips forward attempting to cross motorway and faces head on with a lorry, sending Him 50 metres horizontally

Bloke: oi, fuck you alright mate?
Bloke 2: are you simple? bloody help Him man!