Probably by means of partial understanding. Someone talks about "an awful battle", or "the awful day of judgment", and people (especially children) might under

Probably by means of partial understanding. Someone talks about "an awful battle", or "the awful day of judgment", and people (especially children) might understand it as fearsome and potentially very bad, and use the word exclusively in that sense. It's easy to imagine the definition migrating to simply, "very bad".