To escape many _ in LaTeX efficiently

I couldn't get the underscore package to work, so I used the url package:

\usepackage{url}
\urlstyle{sf}  % or rm, depending on your font

...

Foo \url{word_a_a_a_a_a_b_c_dd} bar.

Are you thinking of the underscore package, which redefines the underscore symbol so that you don't have to escape it in text mode? See here.

Typically you want a monospaced font in such situations, so you can use this:

\verb|word_a_a_a_a_a_b_c_dd|

Other than verbatim I wouldn't know.

Verbatim environment:

\begin{verbatim}
  word_a_a_a_a_a_b_c_dd
\end{verbatim}

Inline:

\verb|word_a_a_a_a_a_b_c_dd|