When and why did "patronize" gain the meaning of condescension versus being a customer?

Solution 1:

It must be remembered that to act as a patron was to be in a superior position to the person one was helping. Largesse could be given or withdrawn at will. Consider Samuel Johnson's definition of a patron:

Is not a patron, my lord, one who looks with unconcern on a man struggling for life in the water, and, when he has reached ground, encumbers him with help? The notice which you have been pleased to take of my labors, had it been early, had been kind; but it has been delayed till I am indifferent, and cannot enjoy it; till I am solitary, and cannot impart it; till I am known, and do not want it. — Letter to Lord Chesterfield

You can see that Lord Chesterton's offer of patronage after Johnson had achieved success was not only spurned, but resented as condescension. But if Chesterfield had offered it earlier, Johnson would very likely have accepted it gratefully.

Read more: http://quotationsbook.com/quote/29572/#ixzz1KMc4zsIS on Quotations Book — Letter to Chesterfield

Solution 2:

Short answer: this is the other way around and it was probably end of 18th century. The main reason these two meanings coexists is that the verb has evolved two times.

First from the social and historical meaning of patron (going back to Roman antiquity), a wealthy man keeping a group of attached men and giving them money and places, financing familiar artists like Maecenas who was the patron of Virgiles. As these lords or rich bourgeois had the habit to interfere in their protégés' works or to moralize them without being interrupted for fear of losing the protection, hence this meaning of patronize.

Second, the word patron began to be used as a very obsequious term for "customer" in the 17th century by city merchants. This word is now quite neutral but used to be a flattery, implying a status and a wealth high enough to be a protector of artists and craftsmen, not merely purchasing items in the shop but subsidizing art. The verb's meaning has faded with the word's.

Solution 3:

etymonline has:

1580s, "to act as a patron towards," from patron + -ize. Meaning "treat in a condescending way" is first attested 1797; sense of "give regular business to" is from 1801.