Boyf is a very informal, mostly British way to say boyfriend — the boy or man you are going out with. It is a clipped, chatty word: you might text “my boyf” to a friend, but you would not write it in a school essay or say it to a teacher. It feels cute and casual, like other short forms people use in speech and messages. Americans are more likely to say boyfriend or bf. Same person; just a smaller, jokier word. It names a romantic partner, not “a friend who is a boy.”