i wasnt here from the beginning, but it's a term of endearment that just stuck around in a lot of earlier fan works that people kinda all ended up doing, consciously or not!! it's quite cute :) as far as 'canon' goes, i don't think he's ever called bg 'cub' in real life, but he's been known to throw a really cute nickname like 'jagiya' and 'baby' in bg's direction every once in a while.