GNU Wget is a simple computer program that retrieves content from web servers, and is part of the GNU Project. Its name is derived from World Wide Web and get, connotative of its primary function. It currently supports downloading via HTTP, HTTPS, and FTP protocols, the most popular TCP/IP-based protocols used for web browsing.
Using wget from behind a proxy
wget uses the "standard" proxy environment variables.
See Proxy settings to know how to set them.
wget provides a proxy authentication feature, here is how it works:
$ wget --proxy-user "domain\user" --proxy-passwd "password" URL
Proxies that use a web (HTML) authentication forms are not covered.
Integration in pacman
To have pacman automatically use wget and the proxy with authentication, place the wget command into Template:Filename, in the [options] section:
XferCommand = /usr/bin/wget --proxy-user "domain\user" --proxy-passwd="password" --passive-ftp -c -O %o %u
Of course you need to set the environment variables in order for that to work.