A dear chap is a bad thing, etymologically speaking.
Once upon a time, markets were called cheaps. That's why there's Cheapside and East Cheap in London. Cheap meant any sort of trade or bargaining or financial push-me-pull-you. If prices were low, it was a good cheap, just as with the French bon marché. If prices were high, it was a dear cheap, as in the Promptorium Parvulorum's:
He byeth in tyme and at hour, so that he hath not of the dere chepe
So a market man, a buyer or a seller, became a chapman. Then chapman dwindled to chap so that in The Beggar's Opera (1728) Peachum can say:
Wife, rip out the Coronets and Marks of these Dozen of Cambric Handkerchiefs, for I can dispose of them this Afternoon to a Chap in the City
By which he does not mean fellow, but customer. However, trade and humanity are woven fine. It is in the nature of Economic Man to view all his fellow fellows through the distorting lens of a shop window. Thus we talk today about a tough customer, even though the tough in question may have no intention of buying.
Similarly, chap drifted from meaning a potential purchaser and, sometime in the Eighteenth Century, became a word for any old fellow. And if you like the chap, he's a dear chap and that's now a Good Thing.
A chapbook was a book sold to a chapman
P.S. Is mine the only dank and disgusting mind to have noticed the possible connotations of chap-stick?