The phrase "worst defense" refers to a situation or team that is not very good at protecting or stopping something from happening. It means they are not effective in defending themselves or others. Full definition
We really can't take for granted how bad our defence is. (justarsenal.com)
Inter Milan have the 6th worst defence in Serie A season, conceding 19 goals in 13 games, and new Nerazzurri coach,... (italianfootballdaily.com)
Too bad a defence like a basket that can't hold water, what is wrong with accepting Luis for Giroud exchange like we did with Alexis / Mkhitarayan deal, this defence will make our effort worthless. (justarsenal.com)