The podictionary word for today is terrific: 
Way back in Latin there was this word terrere.  In fact we still have it in English; terror.  It meant then, as it does now, “fright.”  So her terrible day was literally a “frightful” day, and the terrorist alert must have been “terrifying.” 

By now it should be obvious that her mother’s terrific day stems from the same source; terrifying and terrific are pretty close in spelling and sound. The literal meaning of terrific as used by John Milton in Paradise Lost is “something that can scare the bejesus out of you.”  That Paradise Lost citation is in fact the first one in English and is from 1667.  John Milton is describing a serpent—not the serpent—but one of the creatures of God’s creation.  His serpent sports a hairy mane; which I have to admit does sound a little out of the ordinary.  According to his own words, Milton’s purpose in writing this 80,000 word poem was to “justify the ways of God to men.” 

Now that in itself is a little terrifying isn’t it?  The way that our word of the day terrific came to mean “wonderful” from a root of meaning “terrifying” is as follows.  When John Milton used terrific he did mean “terrifying.”  But by 1800 people were using terrific as an intensifier, along the same lines as my kids might today tell me that a party they were at was so much fun that it was scary.  Here are the first two citations for this intensifier usage:

• I am up to my eyes in business, the extent of which is quite terrific.
• The crowd was immense, and the applause terrific.

You can see from these citations that the meaning could go either way.  The applause or the amount of work could terrify, or it could be wonderful.  But it wasn’t until 1930 that we see the first citation for terrific plainly meaning a very good thing. 

And in that citation there is another example of a similar trend among words that intensify.  The citation goes:

‘Thanks awfully,’ said Rex. ‘That’ll be ripping.’ ‘Fine!’ said Derek Yardley. ‘Great! Terrific!’

That word great had earlier meant “large,” here it means “very good.”  In fact that opening dialogue I started with:
“Terrific!  We just finished a tremendous project, on time, on budget.”
That uses the word tremendous which originally meant something that induced trembling with fear, then followed a similar route to a similar current meaning.