Children are routinely scolded for using this word. Its condemnation is a perfect illustration, and perhaps the perfect one, of the vagaries of usage. The OED records its use in writing as 'an't' by the eighteenth century and as 'ain't by 1778. It is contraction of the same form as 'aren't,' ' isn't,' and 'hasn't', and it makes grammatical sense. Yet at some point it became a taboo word and has remained so.

You can easily imagine an alternative universe in which 'ain't' became Standard and usage pundits condemned native speakers for sloppily failing to use it. Fowler comments sensibly: 'A(i)n't' is merely colloquial, and as used for 'isn't' is n uneducated blunder and serves no useful purpose. But it is a pity that 'a(i)n't' for 'am not,' being natural contraction and supplying a real want, should shock us as though tarred with the same brush. Though 'I'm not" serves well enough in statements, there is no abbreviation but 'a(i)n't I? for 'am I not?..."

A pit indeed. Language doesn't follow logic, except its own, Aren't would be an alternative as in 'aren't clever?, yet I 'aren't clever' isn't standard English