Antonyms
'Awful' once meant awesome
Both words came from awe. Awful drifted to terrible while awesome kept the wonder, splitting a single idea in two.
More in Antonyms
Antonyms come in three flavours
Gradable antonyms sit on a scale like hot and cold, complementary ones exclude each other like alive and dead, and relational ones need each other like teacher and student.
'Flammable' and 'inflammable' mean the same
Inflammable comes from inflame, not from a negative prefix. Safety officials pushed flammable to stop deadly confusion.
'Up' and 'down' break the rules
Slow down and slow up mean the same thing. Idioms often ignore the logic of their own antonyms.