[Tex/LaTex] list item multilined ( `\newline` inside of item label ?)

Question

How to make multiline item label ?

Example

\documentclass[a4paper,10pt]{article}
\usepackage{lipsum}
\begin{document}
\begin{itemize}
\item[Great Item] \lipsum[1]
\end{itemize}
\end{document}

I would love to do

\item[Great \newline Item] \lipsum[1]

But I does not work.

Expected result – first lines of litem label and item content aligned:

Great   Lorem ipsum dolor sit amet,
Item    consectetuer adipiscing elit.
        Ut purus elit, ver...

---

To be more specific I use mydesc environment provided at https://tex.stackexchange.com/a/23569/7128 .
I would love to make it work together.
Above example is overslimplified.
Here is more detailed :

\documentclass[a4paper,10pt]{article}
\usepackage{lipsum}
% mydesc thanks for https://tex.stackexchange.com/a/23569/7128
\newenvironment{mydesc}[1]
  {\list{}{\renewcommand\makelabel[1]{\it##1\hfil}%
     \settowidth\labelwidth{\makelabel{\it#1}}%
     \setlength\leftmargin{\dimexpr\labelwidth+\labelsep\relax}}}
  {\endlist}
\begin{document}
\begin{mydesc}{LongestItem}
\item[Great Item] \lipsum[1]
\end{mydesc}
\end{document}

Answer 1

LaTeX offers a command \shortstack for this kind of thing, so your solution could look like this:

\item[\smash{\shortstack[l]{Great\\Item}}] \lipsum[1]

Unfortunately, \shortstack is bottom-aligned by default, so we have to hack a "top" variant:

\makeatletter
\gdef\tshortstack{\@ifnextchar[\@tshortstack{\@tshortstack[c]}}
\gdef\@tshortstack[#1]{%
  \leavevmode
  \vtop\bgroup
    \baselineskip-\p@\lineskip 3\p@
    \let\mb@l\hss\let\mb@r\hss
    \expandafter\let\csname mb@#1\endcsname\relax
    \let\\\@stackcr
    \@ishortstack}
\makeatother
\begin{document}
\begin{itemize}
\item[\smash{\tshortstack[l]{Great\\Item}}] \lipsum[1]
\end{itemize}

The code for \tshortstack was just copied from the respective code in the LaTeX sources. If you don't want to copy so much code, you can also use \patchcmd:

\documentclass[a4paper,10pt]{article}
\usepackage{lipsum}
\usepackage{etoolbox}
\makeatletter
\gdef\tshortstack{\@ifnextchar[\@tshortstack{\@tshortstack[c]}}
\let\@tshortstack\@shortstack
\patchcmd\@tshortstack\vbox\vtop{}{}
\makeatother
\begin{document}
\begin{itemize}
\item[\smash{\tshortstack[l]{Great\\Item}}] \lipsum[1]
\end{itemize}
\end{document}