To the manner born
Destined to be suited to something, by virtue of birth or custom and practise.
Any examination of 'to the manner born' has to include a mention of its often-quoted incarnation, 'to the manor born'. That has a similar meaning but stresses manorial birth, that is, it refers to someone born into the nobility.
The 'manner' version is earlier and there's some debate amongst etymologists as to whether the second of these phrases was coined deliberately as a play on words, or whether it is just a misspelling of 'manner' as 'manor'. The third possibility, that they arose independently, is highly unlikely.
'To the manner born' was used by, and probably coined by, Shakespeare, in Hamlet, 1602:
HORATIO: Is it a custom?
HAMLET: Ay, marry, is't:
But to my mind, though I am native here
And to the manner born, it is a custom
More honour'd in the breach than the observance.
The meaning there is clear. Hamlet knows the custom being spoken of because he is native, that is, born locally.
Hamlet was written in or around 1600 and published in 1603. The 'manor' version comes much later. The earliest reference I've found so far is in The Times, July 1859, in a story about the Emperor of France's visit to Austria:
"Before Solferino, Austria was only an intruder in Italy; now she is as one 'to the manor born'."
To the Manor Born is the name of a popular BBC comedy series that was broadcast in 1979. The final episode had the highest audience for a single UK TV show at the time, which accounts for some of the penetration of this version of the phrase into the language. The show's scriptwriter, Christopher Bond, would certainly have been aware of the line in Hamlet when he adopted the title and it's clear that he used the 'manner/manor' play on words knowingly.
The question is, did 'to the manor born' originally become adopted because of a play on words or by mistake?
It would appear to be a simple mistake. Some people may have been aware is the 'manner' spelling but it's hardly credible that everyone did. The 'manor' version of the phrase is now far more popular in the language than the earlier one. Examples of its use make it clear that the distinction between 'manner' and 'manor' is now being lost. Given the closeness of the meaning of the two phrases, they have now become virtually interchangeable.
There are many examples of such errors, where words are replaced by plausible, similar-sounding alternatives. In fact, this is so common a linguistic form as to have been given a name - eggcorn. This name was coined in 2003 following the example of a misspelling of acorn as egg corn. If you know that acorns are egg-shaped but aren't sure how the word is pronounced, 'eggcorn' would be a reasonable guess.
'To the manor born' makes sense, as people born in manors could be expected to be born into a life of pre-determined affluence. The existence of the previous phrase 'born with a silver spoon in one's mouth', would also tend to have made the 'manor' phrase intuitive. It may well have been coined by mistake but, given its adoption into the language, it's perhaps overly strict to label it as incorrect. It could now be viewed as an example of how language mutates with time.
See other phrases and sayings from Shakespeare.