 Amn 't as a contraction of am not is known from 1618. As the "mn" combination of two nasal consonants is disfavoured by many English speakers, the "m" of amn't began to be elided, reflected in writing with the new form an 't. Aren't as a contraction for are not first appeared in 1675. In non-rhotic dialects, aren 't lost its "r" sound, and began to be pronounced as an't.
