PIECE DE RESISTANCE

What is the meaning of this phrase? I remember a pub bore telling me it was French for Torture Chamber - "piece" meaning room, and "resistance" referring to the resistance of the person being interrogated, which presumably would be overcome through torture. Is there any truth to this?

No. It is much simpler than that - it means a piece (or article) of (or with) staying power, longevity or the like.

DFG

Your pub bore had come across a hoary joke genre called "Fractured French", and either had failed to realise it was a joke, or was pulling your leg with it. "Fractured French" consists of (mildly) amusing skewed translations of stock French phrases, as in the following:
Coup de grace - lawnmower
Tant pis, tant mieux - Auntie spends a penny, Auntie feels better
J'y suis, j'y reste - I'm Swiss and I'm staying
Moi aussi - I'm Australian.
and so on and so forth. (VSD)

Okay, you guys, can we get back to the table? The most common meaning of pièce de résistance is the main dish at a dinner, or as the OED puts it:
" 1. The most substantial dish of a meal. Now rare.
In quot. 1789 used in fig. context so as to resemble sense 2.
[1797 E. BURKE Lett. Peace Regic. France iv, in Wks. IX. 7 Our appetite demands a piece of resistance.] 1789 R. CUMBERLAND Imposters I. 13 Fair Eleanor is the dish of dainties at the head of the table; plain Dorothy is the piece de resistance at the foot of it. ...

2. The prize item in a collection, group, or series; the climax or most impressive part of an event, display, etc.
1878 Scribner's Monthly Dec. 279/1 In the Austrian Gallery, Mackart's large picture of 'Charles V. entering Antwerp', is the pièce de resistance. 1893 Outing 22 149/2 The pièce de résistance of the entire ride lies between Poughkeepsie and Yonkers. 1958 Observer 9 Mar. 10/4 But do not miss, as you so easily could, the two pièces de résistance. 1995 Mixmag May 19/1 His piece de resistance is a huge wooden box full of placebo drugs."
For most people, I think, sense 1 provides the model for understanding the various uses in sense 2.
SS