I've created that new macro:
\newcommand{\undbf}[1] {\textbf{\underline{\smash{#1}}}}
To get underline closer to letters but, for instance with this code:
\documentclass[8pt,a4paper,dvipsnames]{article}
\usepackage[utf8]{inputenc}
\newcommand{\undbf}[1] {\textbf{\underline{\smash{#1}}}}
\begin{document}
\undbf{StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange, StackExchange,}
\end{document}
I've not got line-breaking automatically; I mean:
And I would like could underline text in that way but have got automatic new lines. Thank you so much :)

soulorulempackage. – egreg Jul 12 '16 at 22:54\smashis wrong anyway – egreg Jul 12 '16 at 23:01