18th century men's "underwear"

Commonly, the long tails of the shirt filled this purpose. There are also underwear constructed like breeches with drawstrings at the waist.

Personally, wear what you want as what is under your clothes is not the business of anyone but yourself...

Breeches linings serve the purpose as do shirts. Drawers mentioned above, though usually only worn for warmth were mostly popularised during the Victorian era to slowly become "underwear". I'm pretty sure breech clouts are outerwear. The point is, why do you need whitie tighties, being that the modem concept of underwear is still rather new and somewhat pointless?

This has always been a bit confusing to me. My general impression after reading a lot of items over the years is in line with what ewoaf said, that there were undergarments, but they weren't "underwear" as we think of it, rather garments worn under the outer clothes to keep warm. There are exceptions which make some of the references seem that they are speaking of outer garments. An assortment of references: 

A list of materials issued to French soldiers and militia in the F&I War,

2. EQUIPMENT FOR THE SOLDIER DURING SUMMER:
1 blanket; 1 capot; 1 cap; 2 cotton shirts; 1 pair of breeches; 1 pair of underpants; 1 pair of leggings (mitasses); 2 skeins of thread; six needles;1 awl; 1 fire steel; 6 musket flints; 1 knife; 1 comb; 1 worm; 1 tomahawk. [Underpants in summer?]

Some modern writer discussing 18th-century underwear:
"However, there is some evidence to suggest that knit breeches and waistcoats might have been more widely made as undergarments, worn under street clothes, especially in winter, in much the same manner as "long johns."

On January 3, 1795 the Fleecy Hosiery Manufactory of L. F. Levallain advertised a wide variety of stockings, socks, gloves, mitts, etc. as being manufactured by them, including "For gentlemen's under-clothing, from No 1 to 3, fleec'd Shirts with and without sleeves, drawers, pantaloons, bosom friends, night caps, ... " [This might have been a knitting factory.]

The Pennsylvania Gazette
December 28, 1769
RUN away….had on, when he went away…. long ozenbrigs trowsers, with white flannel drawers under them, blue ribbed stockings, and country made shoes,

The Pennsylvania Gazette
August 24, 1785
Just imported in the Brigantine Commerce….for SALE, by JOHN SHIELDS…. gauze hose and socks, suitable for wearing under silk stockings;

Are these outer garments?
The Pennsylvania Gazette
March 25, 1731
Had on a good Felt Hat, a Cap under it, greyish Kersey Jacket, Linen Drawers with Pewter Buttons, old grey Stockings much darn'd, and good new Shoes.

Doddtidge seemed to use these terms interchangeably, as outerwear:
"The hunting shirt was generally made of linsey, sometimes of coarse linen, and a few of dressed deer skins. These last were very cold and uncomfortable in wet weather. The shirt and jacket were of the common fashion. A pair of drawers or breeches and legging, were the dress of the thighs and legs; a pair of moccasons [sic] answered for the feet much better than shoes."

Drawers being worn for warmth in July?
The Pennsylvania Gazette
July 15, 1731
RUN away …. From Thomas Green, one named John Parker, about 20 Years of Age, short and well set, brown Hair; has on an old Coat, a new Felt Hat, a new Shirt, with Trowsers, and Drawers under them.

It's enough to get your knickers in a knot.

Spence