There are some mistakes in English that occur because the speaker or writer is trying to make a reference they don’t understand, understandable.
These are called eggcorns.
The most common one you are likely to have heard is “for all intensive purposes”. It is wrong, of course. The phrase is “for all intents and purposes”. But “intensive purposes” sounds as if it might be correct–serious purposes, perhaps, or purposes that should be pursued with unusual determination.
The term “eggcorn” itself is quite recent. In 2003 the American linguist Geoffrey Pullum wrote about a woman who insisted that the correct word for an acorn was “egg corn”. She had never seen the word written down and had simply reconstructed it into something that seemed logical to her. Pullum adopted the example as the name for this whole category of mistake.
At first thought “eggcorn” seems a ridiculous substitution. But once “egg corn” lodges in the mind, the brain starts trying to justify it. Acorns are small and round like eggs. They are also a type of seed or grain. The substitution becomes believable.
Eggcorns join malapropisms and mondegreens as symptoms of misunderstood or misheard English.
A malapropism is when somebody uses a similar-sounding word incorrectly — usually with a comic and catastrophic consequence. Tony Abbott (Australian ex-Prime Minister) rather famously mentioned a suppository of all wisdom when he really meant repository. The press had a field day.
A mondegreen is something misheard, especially in songs. The term comes from a line in a Scottish ballad misheard and transformed by the listener from “and laid him on the green” into “and Lady Mondegreen”.
The important distinction of eggcorns is that they almost make sense. English is full of them:
“Old-timers disease” for Alzheimer’s disease
“Free reign” for “free rein”
“Nip it in the butt” for “nip it in the bud”
“Mute point” for moot point
“Doggy dog world” for dog eat dog world
“Peak my interest” for pique my interest
“Escape goat” for scapegoat
“Tow the line” for toe the line
“Wipe board” for whiteboard
“Card shark” for card sharp
“Damp squid” for damp squib
“Pass mustard” for pass muster
So a mondegreen is a word misheard and transformed; a malapropism is a similar word misused; and an eggcorn is a word misunderstood and repaired into something with a slightly easier but different meaning.
Eggcorns survive because they sound as though they could be right and for all intensive purposes, some of them almost are.