Q. Two examples spring [跳] to mind immediately, the first being: terrible (bad) and terrific (good)
The other was: awful (bad) and awesome (good).
That last one is particularly vexing [烦人] to me. Surely if something with "some" awe is good, something that's "full" of awe would be better, no?
A. Actually, there was a time when terrific was used to denote terrifying. The OED [ Oxford English Dictionary] marks the modern denotation of "marvelous" as beginning around 1930, so this usage was perhaps coined [创造新词; 制造硬币] by the flappers [不拘传统的时髦少女], who were cool, hip [时髦的], and on the ball [alert to new ideas, methods, and trends]. According to the OED, this modern usage, now almost always denoting "marvelous", is a colloquialism. A colloquialism is a colloquial word or phrase, or an expression or word used in informal conversation.
As for awesome, the modern use you've noted is first recorded in the OED in 1980, which probably indicates that the American hippies [嬉皮士] coined this particular use during the 60s and 70s. The OED marks this usage as slang [俚语], though this is now the most used denotation.
As to why this has happened: English is a living language. This means that new words are still finding their way into the English lexicon [词汇], and the denotations [denote: mean, indicate] and connotations [association] of existing words are still migrating [changing meaning] with time. Compare this with a Latin [拉丁文] word - Latin being a "dead" language - the denotation or connotation of which will never change no matter how much time passes.
火星文，趣味地意指地球人看不懂的文字，由符号、冷僻字或汉字拆分后的部分等非正规化文字符号组合而成。随着互联网的普及，年轻网民为求彰显个性，开始大量使用同音字、音近字、特殊符号来表音的文字。由于这种文字与日常生活中使用的文字相比有明显的不同并且文法也相当奇异，所以亦称火星文，趣指地球人看不懂的文字。如：1切斗4幻j，↓b倒挖d！(一切都是幻觉，吓不倒我的！)