In the google "Ngram" search, which allows you to search the usage of words in recent history, I typed in the widely-used internet word "LOL". To my surprise, this came up: 
As you can see, there was a large usage of the word "lol" around 1810, even larger than the usage today. In addition, I set the time frame back all the way to 1500 AD, and this absolute unit came up:
My question is, Does anybody know exactly why the usage of LOL was drastically boosted in the 1600s to the 1800s? Thanks!
In Ngram viewer, along the bottom you can find the actual books that contain the words. By following those links I came across The Changeling By Thomas Middleton, William Rowley whose text looks like this:
So there you have it. "Lol" is a contraction of "Lollio", which looks like a character's name in this book.
In general, I would be extremely wary about Ngram results for
Congusbongus's answer is very good, and points out that you can find the actual books that contribute to the spike: link. I think "higher prevalence of OCR errors, and very low sample population" is a solid hypothesis.
I noticed one more phenomenon of pre-1800 English text that could contribute to this particular OCR error. See, "lol" has two "l"s, and pre-1800 English text has a statistically very high number of "l"s when run through OCR... because of the long s. I see at least two cases of Google reading "lol" in the middle of a word that actually contains "loſ" with an S:
Here, in the middle of a pretty good OCR, Google reads "loſt his horſe" as "lol't his horfe."
Here, grasping at straws, Google reads "cloſe" as "C'lOl'i.'"
But Google also reads "lol" in a couple of places for "fol." (meaning "folio") and for "for." Maybe there's something about these old typefaces that makes their "f"s as well as their "ſ"s look like "l"s.
Graham's theory of "lol" being commonly found in "Fol lol" and "Tol lol lay," while not directly relevant to your 1500–1700 search, is certainly borne out by the next century's worth of ngram results!
kek was higher than I expected, thanks to the legit use of the word in Dryden's Mr. Limberham (1) but mainly thanks to one particular filigree design (2, 3)! And even here we find a mis-OCRed long S: "kek" for "ſeek".
Commented
Dec 25, 2018 at 21:16
In addition to the previous answer...
"Lol" is a nonsense syllable frequently used in mouth music. Other similar syllables are "fa", "la", "fiddle", "diddle", and so on - think of the song Deck the halls, for example, or Whack-fol-de-diddle from the Dubliners. This has always been a staple of English folk music. More recently, scat singing uses the same principle in jazz. The concept is the same though - the nonsense syllables are chosen for their sustained and/or plosive qualities to fit the meter of the song.
In the 16th century, madrigals spawned a new genre called balletto which featured this as a standard part of the form, and this coincided with the widespread availability of printing. It is not inconceivable that transcriptions of broadsheet songs would skew the statistics for these mouth music "words".
"To my surprise, this came up"
Why, you don't honestly believe it was an acronym for laughs out loud in 1800 do you?
You should give a little thought to the idea that what you've searched there includes real words as well as acronyms with completely different meanings that just happen to be spelled the same.
An old or alternative (or just lazy) spelling of loll seems the most likely cause of those statistics to me.
meaning to "sit, lie, or stand in a lazy, relaxed way."
Words don't just fall in & out of favor with time, they change, old words disappear, new words appear & something spelled the same way in 1500 as a word in 2018 may have absolutely nothing to do with the current word either in it's usage & meaning or the etymology of its origins.
Your results from 1500 are pretty pointless in this instance & certainly don't point to the use of lol as an acronym for laughs out loud in that century that you seem to be wondering about.
