Psi is a multi platform and standards based instant messaging client available for Windows, Linux, Mac and more.. . psi lb.p.sq.in. lbs , , ” ” . pound force per square inch, lbf in .Psi XMPP Jabber . .Psi is a cross platform powerful XMPP client designed for experienced users. Psi is a development branch of Psi XMPP client. More details Project Purpose .