(Internet) trackback

(baseball) backstop

backrooms (of a store)

backlash (a strong adverse reaction to a political or social development)

backsword

(dated slang) a woman who is attractive from behind, usually in reference to her buttocks. / (dated slang, in particular) butterface: A woman who is attractive from behind but ugly from the front.

to know through one's own experience

complete denial of knowing (an incident in question)

(Classical Japanese, archaic, honorific) to know

God only knows