Wojak is a Polish word meaning "soldier" or "warrior," most often used in an informal, literary, or poetic context. The diminutive form, Wojaczek, is a common Polish surname.
In internet slang, "Wojak" is the name of a widespread internet meme, also known as the "Feels Guy," which in its original form appears as a crude, black-and-white line drawing of a bald, male head with a slightly wistful expression. The exact origin of the image is unknown, but it seems to have first emerged in 2009 on a Polish imageboard named vichan before later being reposted to the German imageboard krautchan in 2010 by a poster called "wojak," whence it derives its name.
In any case, it soon spread to 4chan where it became wildly popular as an image macro, originally often associated with the phrase "That feel when...," hence the alternative moniker "Feels Guy." The image was easy to modify and was transformed into all sorts of memes, including stock characters in the memescape such as Coomer, Doomer, NPC, and Chudjak.