I was just wondering.
“Peer pressure” is an expression first recorded in English in 1971. I was surprised to learn that. I was in grad school and in my 30s by then and I would have said that it is an expression I had heard all my life. In fact, I think I remember fighting my way through the Gilbert and Sullivan sense of “peer”—Peers will be peers/and youth will have its fling”—just to get to “a jury of one’s peers.”
Maybe not. But I can tell you for sure that when I looked at “pressure” in this expression, it hit me like “relief” in “tax relief.” A Democratically inclined wordsmith pointed out that you are saying something about taxes when you call a reduction in the rate, a “relief.”
In the expression “tax relief” you appear to be saying something about “the lifting of a burden.” In fact, English gets the word from the Latin levare, “to lift up, to lighten.” But the power of the expression is the silent categorization of taxes as the kind of thing from which one ought to desire relief. Taxes are not, in this construction, “the price we pay for civilization.”  “Relief” is a word that instructs us not to look for a benefit, but only at the immediate cost. And that is the power of “relief” in the expression “tax relief.”
What is the power of the word “pressure” in “peer pressure?” It helps us that it is a recent coinage. We still have a grip on ways “pressure” was used back in the 70s. Pressure, for instance, is not persuasion. The reason “make him an offer he can’t refuse” has become so well known is that it sounds like a bargaining situation—that’s the value of the word “offer”—but the narrative shows us that coercion is meant. Similarly a “pressure defense” in basketball makes it very difficult for the offense to operate at all. The “pressure” in a high pressure job has to do with the offer of rewards and the threat of failure.
As I was saying, we are familiar with “pressure” as a general notion. So we introduce pressure as something one’s “equals” might exert.  For our purposes, it doesn’t really matter in what way these others are your equals. The definition in etymonline.com says that when it was introduced around 1300, it meant “equal in rank, character, or status.” If fact, the images that a search turns up all have to do with teenagers as if adults no longer have to take account of the views of their peers. [This set of two images suggests the product of the search. Note that there is nothing remotely like pressure in the left hand image and that the right hand image relies on cigs. Oooh.]
The value, I think in looking at the expression “peer pressure” as an idea is all the other things it excludes. It excludes instruction, for example. If you want to do what the others are doing and don’t really know how, it would be very helpful if someone would teach you. There is no pressure in this instruction; no implicit threat.
It excludes emulation too. I was very nervous about starting doctoral studies—as one might be who escaped his undergraduate institution with very low GPA and who was admitted to a masters’ program on probation—and if I had showed up at Oregon and discovered that all the other male members of the group were wearing beards, I would have grown a beard as fast as I could push the hairs out of my chin. That is true even if they were very generous and accepting to beardless new members. I would have wanted very much to look like them and not shaving for awhile is the easy way. 
That is emulation. Entirely apart from any negative sanctions  you are attracted to the way your peers are. That is another meaning you walk by when you accept “peer pressure” as the kind of influence peers exert.
 Thank you Oliver Wendell Holmes, Jr. Here is a longer (and better) version of the quotation: “It is true, as indicated in the last cited case, that every exaction of money for an act is a discouragement to the extent of the payment required, but that which in its immediacy is a discouragement may be part of an encouragement when seen in its organic connection with the whole. Taxes are what we pay for civilized society, including the chance to insure.”
 The English “peer” derives from the Latin “par,” equal.
 There is a very funny spoof of “the beatnik culture” in the movie What’s so Bad About Feeling Good?” These beatniks are determined to reject the values of the majority culture (bourgeois) and they reject it in lockstep, identically.
 Have you noticed that although you used to have to be careful to distinguish positive from negative sanctions, you don’t have to any more. “Sanction” now means “negative sanction” and if you want to mean something else, you have to specify and then swim upstream for awhile.