In slang, BMT can be an acronym for Big Man Ting, a Jamaican English phrase meaning "grown-up thing" and referring to adult matters or concerns. Jamaican speakers may use big man ting with the effect of "for real," "seriously," or "no joke," e.g., Big man ting, that new single from Drake is the greatest. A related phrase is big people ting.
BMT was popularized by British rapper Fredo's 2018 song "BMT." Fredo's music is influenced by grime, a music genre informed by Jamaican reggae and the Jamaican diaspora in South London.
In internet slang, BMT is an acronym for biting my tongue. In other places, such as Twitter, BMT often refers to Bone Marrow Transplant. It also stands for Basic Military Training.