Many words that have long been considered obsolete, return to our language. Therefore, it is important to understand the appropriateness of using different words and to know all the interpretations of each expression. For example, the adverb is "fairly": the meaning of a word is accessible to almost every literary connoisseur, but it can be used in different sentences and change its interpretation.
Interpretation of the word
• a lot of;
The last interpretation is knocked out of the given series, since in this sense the word has not been used for a long time.
Fairly: the meaning of the word in the fable of Krylov
As you have already noticed, there can be several alternatives of this expression. However, not all the possible options are given above. When you study the adverb "fairly", the meaning of the word will not be fully defined if you do not associate it with the adjective "hefty." In turn, this adjective is interpreted as follows:
• significant for any characteristics (for example, strength or size);
• good or excellent.
Hence it can be concluded that in some cases "fairly" can mean the same thing as well. In Krylov's work "The Donkey and the Nightingale" this dialect is used just by analogy with the adjective above. A donkey saying Nightingale that he is pretty sings, appreciated the sounds produced by this bird as good, pleasant rumor.
How appropriate is it to use this word in modern speech?
Such a substitution will be understandable to readers or interlocutors and it will not become unpleasant to stand out. Perhaps you have already met this word in the meaning of "a lot", but did not pay attention to it.