John is correct. Most adjectives are gradable, which means they can be described at different levels of intensity (e.g. "Very happy, "a little happy", "somewhat happy"). Other adjectives are non-gradable, which usually can't be described in such a way. For example, someone is either pregnant or they aren't, so "She's a little pregnant" doesn't make sense.
In the case of words like "terrific" or "amazing", words like "really" or "very" are already built into the definition ("terrific" = "very good"), so adding them sounds awkward.
One caveat: sometimes speakers will intentionally break this rule for emphasis or humor. For example, "very pregnant" is grammatically incorrect, but some speakers will say to emphasize that someone is in the final weeks of her pregnancy ("A very pregnant woman like her shouldn't be exercising that hard!").