What about thieves' cant? Here, let me share a song:
The Ruffin cly the nab of the Harmanbeck,
If we mawnd Pannam, lap, or Ruff-peck,
Or poplars of yarum: he cuts, bing to the Ruffmans,
Or els he sweares by the light-mans,
To put our stamps in the Harmans,
The ruffian cly the ghost of the Harmanbeck
If we heaue a booth we cly the [Jerk].
This is from The Beggar's Curse (1608), described by a contemporary (Dekker) as "a canting song, wherein you may learn, how this cursed generation pray, or (to speake truth) curse such officers as punish them" (see the book Three Centuries of Canting Songs and Slang Rhymes [1536 - 1896] for this and more).
A tiny bit of the barrier to understanding it is just it being old. For example, I'm sure you know the words "els" (else), "sweares" (swears), and "heaue" (have).
And then there's the cant, which is almost the rest of it, because the very point of cant is to obscure the meaning of what's being said:
- The Ruffin/ruffian cly [someone]: the devil take [someone] (used to wish evil on someone)
- nab: head
- Harmanbeck: a beadle
- mawnd: beg or ask
- Pannam: bread (related to Latin panem)
- lap: liquor (or some other drink)
- Ruff-peck: bacon
- poplars of yarum: milk pudding
- cuts: says
- bing to the Ruffmans: off to the hedges
- the OED lists "bing" as "Slang. ? Gipsy." and not cant specifically
- light-mans: day
- stamps: legs
- Harmans: the stocks
- This and several other words use the suffix "man(s)", which is used "either as an affectionate diminutive or as a disguise device" according to the OED
- cly: take or steal
- Jerk: a seal on a counterfeit license (the counterfeit license itself is called a "gybe" in cant)
- While Gutenberg has this as "lerk" (starting with an L), this is certainly an error for either "Jerk" or "Ierk", which were just two different ways to write the same word
See also Green's (2015) and Dekker's (1608) translations of the song.