"The Partridge Family" star David Cassidy is learning the hard way ... it's never a good idea to stiff your lawyers, no matter how dreamy you are (allegedly) ... because they'll sue your bell-bottom pants off.
David -- aka the Justin Bieber of his day -- is being sued by Rodier & Rodier law firm, which claims David racked up $134,221.50 worth of legal bills from 2006 to 2011 ... but never fully paid them off.
In fact, the lawyers say the former teen idol has only coughed up $25,000 to date -- and now they're asking the court to force David to fork over the remaining $109,221.50 ... plus interest.
A rep for David tells TMZ ... "There seems to have been a misunderstanding between them which I believe will be amicably resolved."
We're guessing with a check.