2535: Why 'Un-' Doesn't Assimilate Like 'In-' (Ir- Il- Im-) Nov 21, 2021

The prefix 'in-' assimilates to different sounds depending upon the context, becoming 'im-', 'il-', and 'ir-', but this is not true of 'un-', which is similar both semantically but more importantly, phonetically. For instance, 'in-' + 'responsible' is 'irresponsible', but 'un-' + 'responsive' is 'unresponsive'. This was true of the negating prefix 'in-' in Latin whence this use was borrowed, and while 'un-' is distantly related to this, it is really considered to be of a Germanic root where this does not happen.