Green’s Dictionary of Slang

b.u. n.

[abbr. biological urge]

(US campus) sexual drive or desire.

[US]J.T. Farrell ‘Meet The Girls!’ in Short Stories (1937) 267: Her body was just B.U. tiful.
[US]G. & S. Lorimer Men Are Like Streetcars 5: They’re all alike [...] when you stir up the old B.U. [HDAS].