The second letter of the alphabet, the 10th letter of the alphabet... if you are unfamiliar with that, here's the alphabet, with the second and tenth letters capitalized...
a B c d e f g h i J k l m o p q r s t u v w x y z.
If you are still that daft, try this:
or Blow Job.
<Girl> Oooh baby! How about a 2-10?
<Camel_Kid> Fuck yeah!