Something that is silly is absurd, ridiculous, or irrational—but not always in a negative way. Sometimes the word silly suggests amusement or even endearment (you silly goose!). Frivolous, on the other hand, almost always suggests a lack of value, merit, or necessity. A frivolous person is someone who is self-indulgently carefree and does not engage seriously with matters. A frivolous lawsuit is one that lacks legal merit and is not to be taken seriously.
Time is of the essence when it comes to these two adjectives, so we'll get right to it: quick describes a speedy tempo; expeditious emphasizes promptness and efficiency, especially in the completion of a task. An expeditious answer to an inquiry is one that takes no time at all. Expeditious is easy to confuse with expedient, but the latter (expedient) is better used to describe things that are suitable or advantageous for a specific purpose, such as politically expedient statement.
To belie is to create an impression that is inconsistent with the facts or that contradicts evidence. For instance, if a friend is feeling very nervous and insecure about a presentation but somehow manages to give off an aura of nonchalant confidence while delivering it, you could say their casual, relaxed manner belies their insecurity. The core idea of belie is one of misrepresentation, though the term does not usually suggest an intent to deceive; rather it used to point out notable or striking variances or contradictions.