Learn English – How and why have some words changed to a complete opposite

early-modern-englishetymology

One example that comes to mind is terrific which originally denoted something quite terrorising while now it has positive connotations… How and why did these changes occur?

Best Answer

See Etymonline:

1660s, "frightening," [...]. Weakened sense of "very great, severe" (e.g. terrific headache) appeared 1809; colloquial sense of "excellent" began 1888.

So the transition was gradual in this case, unlike with many slang words that are used to mean the opposite thing on purpose.