A fuck boi is that one 15 year old boy you have on facebook who posts selfies of himself pouting with more than one filter applied over the picture and gets way more like than he deserves.
Typically fuck bois have gelled quiffs, think that it is acceptable for a boy to pout in photos and frequently post selfies with their 'six pack' in full display.
They have over 100 likes on their profile picture because they have 2,000 girls as friends facebook and simultaneously flirt with them all, have smoked weed twice yet own HUF socks and post pictures of their Nikes on instagram with the heart eyes emoji as the caption.
Fuck bois often think that they are cool for bragging about having sex or getting drunk.
"this fuck boi needs to stop writing soppy paragraphs on my facebook newsfeed, everyone knows he's only doing it to get into the pants of one of the 2,000 girls he's friends with"