"seggs" is sexual slang. In short, it refers to algospeak respelling of "sex," used to evade content moderation on tiktok and other platforms. Among teens it is usually used jokingly, as crude banter, or in TikTok-style algospeak rather than as a serious statement of behaviour. Persistent or aggressive use directed at a specific person, especially online, is the bit worth paying attention to: that can shade into harassment. If you hear it once in a chat with friends, it is probably nothing. Repeated targeted use deserves a conversation about respect.