Column headings on the table border

• Typeset the table normally, including the headers.
• Add \tikznode commands for each header 'on the line'.
• Wrap the whole tabular into a \tikznode command.
• Add a tikzpicture to draw the lines.

The \tikznode command is defined and described in this answer to "How to add arrow in equations and matrix".

For fine-tuning, use the optional argument of \tikznode to supply options to tikz. E.g., for adding more space around the tabular and the headers, you can define two styles by

\tikzset{tab/.style={inner sep=2pt},hdr/.style={inner xsep=4pt}}

and add tab and hdr as options to the \tikznode commands.

\documentclass[border=5pt]{standalone}
\usepackage{tikz}
\newcommand\tikznode[3][]{%
  \tikz[remember picture,baseline=(#2.base)]
    \node[minimum size=0pt,inner sep=0pt,#1](#2){#3};%
}
\tikzset{tab/.style={inner sep=2pt},hdr/.style={inner xsep=4pt}}
\begin{document}
\tikznode[tab]{singplu}{%
  \begin{tabular}{llll|lll}
    \multicolumn{4}{c}{\tikznode[hdr]{sing}{SINGULIER}} &  \multicolumn{3}{c}{\tikznode[hdr]{plu}{PLURIEL}} \\
    & & & & & \\
    & MASCULIN & FÉMININ & NEUTRE & MASCULIN & FÉMININ & NEUTRE \\
    \textsc{Nom.} & \emph{is} & \emph{ea} & \emph{id} & \emph{ei} ou \emph{ii} & \emph{eæ} & \emph{ea}
  \end{tabular}%
}%
\begin{tikzpicture}[remember picture,overlay]
  \draw (singplu.south west) |- (sing);
  \draw (sing) -- (plu);
  \draw (plu) -| (singplu.south east);
  \draw (singplu.south east) -- (singplu.south west);
\end{tikzpicture}

\end{document}

Here is a solution with {NiceTabular} of nicematrix.

\documentclass{article}
\usepackage[french]{babel}
\usepackage{lmodern}
\usepackage{nicematrix}
\usepackage{tikz}

\begin{document}

\newcolumntype{:}{!{\vrule}}

\begin{NiceTabular}{|llll:lll|}
\Hline
\noalign{\vspace{4mm}}
  & MASCULIN & FÉMININ & NEUTRE & MASCULIN & FÉMININ & NEUTRE \\
  \textsc{Nom.} & \emph{is} & \emph{ea} & \emph{id} & \emph{ei} ou \emph{ii} & \emph{eæ} & \emph{ea} \\
\noalign{\vspace{1mm}}
\Hline
\CodeAfter
\tikz 
\draw (row-1-|col-1) -- node [fill=white] {SINGULIER} (row-1-|col-5) 
      (row-1-|col-5) -- node [fill=white] {PLURIEL} (row-1-|col-8) ;
\end{NiceTabular}%

\end{document}

You need several compilations (because nicematrix uses PGF/Tikz nodes).